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Appl Environ Microbiol. Jan 2003; 69(1): 41–48.
PMCID: PMC152469

Use of Real-Time Quantitative PCR for the Analysis of [var phi]LC3 Prophage Stability in Lactococci

Abstract

Bacteriophages are a common and constant threat to proper milk fermentation. It has become evident that lysogeny is widespread in lactic acid bacteria, and in this work the temperate lactococcal bacteriophage [var phi]LC3 was used as a model to study prophage stability in lactococci. The stability was analyzed in six [var phi]LC3 lysogenic Lactococcus lactis subsp. cremoris host strains when they were growing at 15 and 30°C. In order to perform these analyses, a real-time PCR assay was developed. The stability of the [var phi]LC3 prophage was found to vary with the growth phase of its host L. lactis IMN-C1814, in which the induction rate increased during the exponential growth phase and reached a maximum level when the strain was entering the stationary phase. The maximum spontaneous induction frequency of the [var phi]LC3 prophage varied between 0.32 and 9.1% (28-fold) in the six lysogenic strains. No correlation was observed between growth rates of the host cells and the spontaneous prophage induction frequencies. Furthermore, the level of extrachromosomal phage DNA after induction of the prophage varied between the strains (1.9 to 390%), and the estimated burst sizes varied up to eightfold. These results show that the host cells have a significant impact on the lytic and lysogenic life styles of temperate bacteriophages. The present study shows the power of the real-time PCR technique in the analysis of temperate phage biology and will be useful in work to reveal the impact of temperate phages and lysogenic bacteria in various ecological fields.

Lactic acid bacteria (LAB) have a long tradition of being used as starter cultures for the fermentation of a variety of food products. In the dairy industry, the species Lactococcus lactis has an important role as a starter for the production of various cheeses. Unfortunately, L. lactis is sensitive to bacteriophage attack, an event that may result in impaired quality of the product because of fermentation failure. Consequently, phages are of great economic significance (6, 10, 31, 36).

Studies have shown that lysogeny is common among lactococci (reviewed in references 11, 12, and 22), and recently the presence of six prophages in the genome of L. lactis IL1403 has been reported (7). The impact of lysogenic starter strains on fermentation processes is not fully understood. However, temperate phages and prophage-derived sequences of the P335 species have shown to give rise to new lytic phages (4, 13, 33), and the spontaneously induced temperate phages have the potential to infect and lyse or lysogenize sensitive strains in a mixed starter culture in cheese plants (11, 18, 23, 38). Furthermore, prophages of LAB are found to be involved in the lysis of starter cells during cheese ripening, in which release of desirable enzymes into the surroundings is an important step in flavor development (19, 24, 35).

Despite the increasing knowledge about the molecular basis for the lysis-lysogeny decision of temperate phages, there is still little information available on the influence of the host and environmental conditions (32, 37). Such studies would be beneficial for optimization of starter culture performance in the dairy industry. The growth temperature, multiplicity of infection, and nutritional resources are factors known to influence the choice of life cycle of the thoroughly studied Escherichia coli temperate bacteriophage λ, which is used as a paradigm for lysogeny in temperate bacteriophages. Low temperature, high multiplicity of infection, and starvation of the cells favor lysogenic development of λ (14, 37). With regard to LAB phages, studies have shown that a shift in temperature (from 30 to 40°C) leads to induction of prophages in several lysogenic L. lactis subsp. cremoris strains (15) and that the spontaneous induction rate varies greatly between lysogenic strains (from 10 to 106 PFU ml−1) and depends on the indicator strain employed in the plaque assay (38). The low number of such studies of LAB might be due to the lack of suitable methods for identifying phages and monitoring phage numbers. In our laboratory an improved plaque assay technique has proven to be useful in identifying indicator strains (25), and we have reported a new method, based on quantitative analysis of specific DNA sites (i.e., the attB, attP, attL, attR, and cos DNA sites) involved in various cut-and-join events in the two life styles of the phage, that is useful for monitoring the lytic and lysogenic pathways of temperate phages (29).

The temperate phage [var phi]LC3 was obtained from fermented milk, and it belongs to the lactococcal P335 group of small isometric-headed phages (27). Characterized genetic elements of phage [var phi]LC3 include the lysA and lysB genes (1), the int gene and the attachment sites for site-specific integration and excision (the attB, attP, attL, and attR sites) (26, 28), the cos region (2), the repressor (orf286) gene (3), and the gene encoding the Cro analogue (orf76) (J. M. Blatny, unpublished data). Sequence comparisons of the lysogeny module reveal that phage [var phi]LC3 shows homology to other P335 phages (3).

The aim of the present study was to use [var phi]LC3 as a model phage to analyze the effect of growth temperature and host strains on prophage stability in lactococci by the use of the method of Lunde et al. (29) and furthermore to develop a real-time PCR assay for quantification of the specific DNA sites involved in this method. The prophage stability of phage [var phi]LC3 was analyzed during growth in six L. lactis subsp. cremoris strains lysogenic for [var phi]LC3 at two different temperatures.

MATERIALS AND METHODS

Bacterial strains, bacteriophages, and culture conditions.

The bacterial strains and bacteriophages used in this work are listed in Table Table1.1. Isolation of the lysogens was done as described by Lillehaug (25). All strains were grown in M17 broth (Oxoid) containing 5 g of glucose per liter instead of lactose. Overnight cultures grown at 30°C were used as 1% inocula, and the cultures were further incubated at 15 and 30°C. To fully adjust and stabilize the bacterial metabolism to the two growth temperatures, the cultures were inoculated every day into new medium adjusted to 15 and 30°C, respectively, and analyses of the cultures were performed on the third day. Growth of the seven lysogenic strains was monitored by measuring optical density at 600 nm with a model 252 colorimeter (Ciba Corning Diagnostics Ltd., Suffolk, England). The study was repeated at least three times, and the data shown are from one representative experiment.

TABLE 1.
Bacterial strains and phages

DNA template preparation.

During growth of L. lactis subsp. cremoris IMN-C1814, samples were withdrawn at regular intervals to perform quantitative DNA analyses (see below) on the cell fraction. For the remaining five lysogenic strains, DNA was isolated from samples harvested at three chosen points at the end of the exponential growth phase and in the early stationary phase. For DNA isolation, samples of 1 ml were snap frozen in liquid nitrogen, centrifuged, and resuspended in lysis buffer (10 mM Tris [pH 8], 1 mM EDTA [pH 8], 1% Tween 80). Further isolation was performed by using liquid nitrogen and boiling water in a freeze-thaw method as described by to Utåker and Nes (39).

Plaque assay.

The number of infectious phages present in the supernatant during growth of IMN-C1814 was determined as follows. One milliliter of bacterial culture were harvested at regular intervals and centrifuged at 7,000 × g for 15 min. The pellet was washed with 0.5 ml of saline containing 10 mM MgCl2, and the cell suspension was centrifuged for 10 min. The pooled supernatant fraction was used for phage titration by a modified plaque assay as described by Lillehaug (25), using L. lactis IMN-C18 as a host.

PCR amplification.

PCR amplifications of the repressor gene orf286 in phage [var phi]LC3 (3) and the gene in phage TP936-1 that corresponds to orf4 (repressor) in TP901-1 (30) were performed with the Taq DNA polymerase kit (Qiagen, Hilden, Germany) according to the instructions of the supplier. PCR amplifications were performed in a total volume of 50 μl and by the use of 30 cycles in a Mastercycler Gradient thermocycler (Eppendorf, Hamburg, Germany). The thermal cycling parameters used were a 3-min hot start at 97°C, followed by denaturation for 30 s at 94°C, annealing for 30 s at 58°C, and extension for 2 min at 72°C. The primers used for amplification of the repressor genes are shown in Table Table22.

TABLE 2.
Summary of PCR primers and TaqMan probes used in this study

DNA sequencing.

DNA sequencing of the L. lactis IMN-C18 region containing the attB site (see below) was performed by the cycle-sequencing dideoxynucleotide chain termination method (ABI Prism BigDYE kit; Applied Biosystems, Foster City, Calif.). Samples were analyzed with an ABI Prism 377 DNA sequencer (Applied Biosystems). The primers used for nucleotide sequence determination were as follows: attB-H1, 5′-TAGTCCTTTTCTTCATTCCTTTTGTTTG-3′; attB-H2, 5′-GACCATAAACCAAGCAAAAGC-3′; attB-H3, 5′-TGGGGTAGATGCCCAC-3′; attB-H4, 5′-CCTAGGCCTTCGTGTGCTTAGG-3′; attB-V1, 5′-GGAGCGGGGAATTATTATAGT-3′; attB-V2, 5′-TCCCATGAAGAATAACAAGGT-3′; attB-V3, 5′-TCCTTACGAACAGATAGAATGGC-3′; and attB-V4, 5′-GCGTGTCACATTCTCTTCGAC-3′.

TaqMan probe design.

The primers and TaqMan probes for real-time amplification of the attB, attP, attL, and chr DNA sites were constructed by the use of Primer Express version 1.0 (Applied Biosystems). The sequences and localizations of the primers and TaqMan probes used are given in Table Table2.2. The primer and probe sets for attB, attL, and attP were designed such that the respective amplification product spans the 9-bp core sequences (26) of the specific DNA attachment sites. The chr DNA site was located just upstream of the attB-core region; it was involved in this assay as an endogenous control to adjust for possible differences in DNA sampling and DNA isolation efficiency and was used to normalize the levels of the attB, attP, and attL DNA sites. The probes were labeled with a reporter dye (6-carboxyfluorescein) at the 3′ end, with a quencher dye (6-carboxytetramethyl-rhodamine) at the 5′ end, and with a phosphate blocking group at the 3′ end to prevent extension by AmpliTaq Gold DNA polymerase.

Detection and quantification of PCR products.

Real-time quantitative PCR was performed by use of the ABI Prism 7700 sequence detection system (Applied Biosystems) on the attB, attP, and attL DNA sites and a chromosomal marker (chr) in DNA extracts obtained from the lysogenic cultures. All reagents used to perform real-time PCR detection were purchased from Applied Biosystems. The amplification reactions was carried out with TaqMan Universal PCR Master Mix containing AmpliTaq Gold DNA polymerase, AmpErase uracil-N-glycosylase, deoxynucleoside triphosphates with dUTP, and passive reference and optimized buffer components. The final concentrations of each oligonucleotide in the PCR mixture were as follows: attB and chr, 300 nM (each) forward and reverse primers; attL and attP, 900 nM (each) forward and reverse primers; all templates, 175 nM TaqMan probe. The total volume of the PCR mixtures was 25 μl, including 1 μl of DNA template, in GenAmp Optical tubes, and the PCR was carried out according to the instructions of the manufacturer. PCR assays for constructing standard curves were performed in triplicate, while the remaining PCR assays were performed in duplicates, both in three separate replicate runs. The concentrations of the primers and probes were optimized as described in the TaqMan Universal PCR Master Mix Protocol (Applied Biosystems).

The increase in emission intensity was measured during the amplification reaction by the ABI PRISM 7700 sequence detection system. Data acquisition and analysis were handled by Sequence Detector version 1.6 software (Applied Biosystems). Briefly, a threshold was assigned to the log phase of product accumulation. The point at which the threshold crosses the amplification curve is defined as a cycle threshold value, termed CT. With increasing target quantity in the PCRs, the CT value decrease linearly, and thus CT values can be used as quantitative measurements of the input target amount (17).

The comparative CT method (user bulletin 2, ABI PRISM 7700 sequence detection system; Applied Biosystems) was used for quantitative analyses, with the following modification: instead of using only one measurement to calculate the relative amounts of the attB, attL, and attP DNA sites compared to chr, we have used the regression functions described in “Statistical and mathematical analysis” below. Time point 8 during growth of L. lactis IMN-C1814 at 30°C (see Fig. Fig.2A)2A) was chosen as a calibrator in the comparative CT method, in which the spontaneous induction frequencies at the other time points were decided relative to the induction frequency at this point. Standard curves were constructed with the DNA sample obtained from the calibrator.

FIG. 2.
Spontaneous prophage induction frequency (attB) and level of extrachromosomal phage DNA (attP) monitored during growth of the lysogenic L. lactis subsp. cremoris IMN-C1814 at 30°C (A) and 15°C (B) by a real-time quantitative PCR method. ...

Statistical and mathematical analysis.

The statistical analyses were performed using the software package SAS 8.2 (SAS, 2001). In order to validate the comparative CT method (user bulletin 2, ABI PRISM 7700 sequence detection system; Applied Biosystems), the amplification efficiencies of the amplicons (attB, attP, attL, and chr) were investigated by linear regression analysis of the CT values of three replicate PCRs for each of three replicate 10-fold dilutions of DNA isolated at time point 8 during growth of L. lactis IMN-C1814.

In order to calculate the efficiency of the amplification reactions and the ratios between the different specific DNA markers and the chromosomal marker (chr), the following equations were used. Equation 1 describes the PCR in the exponential phase (40):

equation M1
(1)

where N1 is the amount of PCR product after n cycles of amplification, N0 is the amount of DNA template prior to amplification, and E is the efficiency of the reaction per cycle. The overall efficiency of the exponential reaction can be calculated from equation 2 (resolved from equation 1):

equation M2
(2)

where E is the efficiency of amplification per cycle and N1N0 is the number of amplified molecules produced in n cycles of exponential amplification.

The ratio between the attachment sites and the chromosomal control was estimated by using the calculated amplification efficiency (E) and the ΔCT (see above) between the attachment sites to the chr DNA site in the following equation:

equation M3
(3)

where i is the DNA attachment site attB, attP, or attL; chr is the chromosomal target DNA; E is the efficiency of amplification per cycle; and ΔCT is the distance (number of cycles) between the intercepts (see below) of the regression functions of the specific attachment sites and the chromosomal control (chr). The intercept (β0) is the value of y when x equals zero in the regression function (equation 4) in which a straight line relating two quantities y and x can be described as follows:

equation M4
(4)

where y is the response variable, x is the predictor variable, β0 is the intercept, and β1 is the regression coefficient (34).

Nucleotide sequence accession number.

The nucleotide sequence of the L. lactis subsp. cremoris IMN-C18 region that contains the [var phi]LC3 integration site (attB) has been deposited in the GenBank database under accession no. AY151819.

RESULTS AND DISCUSSION

We have studied the effect of growth temperature on the prophage stability of the temperate lactococcal phage [var phi]LC3 in six lysogenic hosts by using the recently reported method of Lunde et al. (29). This method is based on the quantitative analysis of specific DNA sites involved in various cut-and-join events during the lytic and lysogenic life styles of the [var phi]LC3 phage, such as the attachment sites for site-specific integration and excision (i.e., the attB, attP, attR, and attL DNA sites) and the phage cohesive ends (cos DNA site). A real-time PCR assay was developed to perform the quantitative analysis of the specific DNA sites used in this work.

Validation of the real-time PCR assay.

Relative quantification of the attB, attP, and attL DNA attachment sites by real-time PCR was performed with reference to the total amount of bacterial DNA represented by a chromosomal DNA fragment here called chr. A previously reported comparative CT method (see Methods and Materials) was used for this analysis. In order to apply this method, it is important that the templates to be compared have equal amplification efficiencies, and in order to verify that this condition was fulfilled, standard curves for the attB, attP, attL, and chr DNA sites were constructed by the use of regression analysis of three replicate 10-fold serial dilutions of a DNA sample. The CT measurements (see Materials and Methods) were performed on different days, and they were highly reproducible, with a standard deviation of less than 0.24 (Table (Table3).3). Therefore, the mean CT value of the three replicates for each dilution factor for each template was used in the analysis below, in which two statistical tests were carried out. When comparing the regression functions of the four templates in the first test, no significant differences between the regression coefficients (P = 0.64) were found, but the intercepts differed significantly (P < 0.001). By the use of multiple comparisons in the second test, it was shown that all templates differed in the intercept except for that of attL and chr (P = 0.06). Therefore, a common regression coefficient (−3.502) but different intercepts (attB, 20.35; attP, 15.19; attL, 13.75; and chr, 13.51) (attL and chr were not significantly different) were used for construction of the four standard curves in Fig. Fig.1.1. The regression functions had a coefficient of determination of greater than 0.98. These results indicated that the efficiencies of the amplification reactions of the four templates were equal, and by using equation 2 (see Materials and Methods), where n was replaced by the common regression coefficient of the regression lines, the amplification efficiency was calculated to 0.93. Furthermore, the standard curves in Fig. Fig.11 showed a linear relationship between the amount of input DNA and the CT values for the four templates over a range of 5 to 6 log units. The large dynamic range is one of the advantages of the real-time quantitative PCR method over other methods for quantitative analysis of DNA fragments, along with the facts that the method involves no post-PCR step, offers high throughput of samples, avoids the use of radioactivity, and requires relatively little manual input.

FIG. 1.
Evaluation of amplification efficiency and range of dynamic amplification of the attB-, attP-, attL-, and chr-specific DNA templates. The mean CT values of attB ([filled square]), attP (○), attL ([open triangle]), and chr ([filled lozenge]) from three serial log ...
TABLE 3.
Average CT values from three standard series of log10 dilutions of a DNA sample used as a calibrator in the comparative CT method

[var phi]LC3 prophage stability varies during growth of the host cell.

The spontaneous induction of prophages in lysogenic bacteria, including lactococci, has traditionally been determined by lysis of bacterial cultures and enumeration of the phages by the use of plaque assays (38). However, the plaque assay has several limitations. The efficiency of plaque formation is highly influenced by the physical and chemical conditions applied with the double-agar plate technique (25). Furthermore, the lack of suitable indicator strains has been a major barrier for studying the temperate bacteriophages. In this work the spontaneous induction frequency of the lactococcal prophage [var phi]LC3 was determined directly on genomic DNA obtained from the lysogenic bacterium by measuring the level of reestablished attB sites on the bacterial chromosome. Furthermore, the phage DNA propagation in the lytic state was elucidated by measuring the level of the attP DNA sites on the extrachromosomal phage genomes in either the linear, circular, or concatamer configuration. The amount of integrated prophages was detected by amplification of the attachment site attL located in the junction between the phage and host DNA, and hence the level of attL DNA sites represented the amount of the [var phi]LC3 lysogenic bacteria in the cultures.

Determination of the spontaneous induction frequency of [var phi]LC3 (attB) and the level of newly synthesized phage DNA (attP) during growth of the [var phi]LC3 lysogenic L. lactis IMN-C1814 strain at 30 and 15°C was carried out by the use of the comparative CT method as described in Materials and Methods, and the results are shown in Fig. Fig.2.2. Time point 8 at 30°C (Fig. (Fig.2A,2A, 8 h) was chosen as the calibrator, and genomic DNA obtained from this point was used to analyze the amplification efficiencies of the various genetic markers (see above). The spontaneous induction rate and the level of the extrachromosomal phage DNA in the calibrator were determined from the ratio of attB and attP DNA sites to the total amount of bacterial chr DNA, respectively, by using the intercepts of their respective regression functions and equation 3 (see Materials and Methods). These ratios (attB/chr and attP/chr) were estimated to 0.0112 and 0.331, respectively. This implies that 1.12% of the cells in the bacterial culture at time point 8 had induced their [var phi]LC3 prophage and reestablished their attB site and that 33.1% of phage genomes (attP) were present extrachromosomally compared to the total amount of the bacterial chromosomal DNA (100%) (chr). According to the 1.12% of attB sites formed, only a small reduction of attL sites (from 100% to 98.9%) should have taken place, and such a small decrease in attL sites could not be determined significantly by our technology.

During growth at both temperatures (Fig. (Fig.2),2), most of the lysogenic bacterial population remained lysogenic, while a fraction of the prophages were spontaneously induced into the lytic pathway. Remarkably, the spontaneous induction frequency did not remain at the same level during the course of cell growth but increased in the exponential growth phase and reached a maximum level in the early stationary phase, as implied by the fact that the development of the attB and attP DNA sites followed the kinetics of the growth curve (Fig. (Fig.2).2). At 30°C (Fig. (Fig.2A)2A) the levels of the genetic markers were 0.4 to 0.6% attB and 5 to 9% attP in the mid-exponential growth phase and increased to averages of 1.3% attB and 30% attP in the end of the exponential growth phase and the early stationary phase. Correspondingly, at 15°C (Fig. (Fig.2B)2B) the levels of the attB and attP DNA sites were 0.4 to 0.6 and 8 to 15%, respectively, in the mid-exponential growth phase and increased to averages of 1.6 and 40%, respectively, in the early stationary phase. At 30°C (Fig. (Fig.2A)2A) a slow decline in the relative concentrations of both attP and attB was observed in the stationary phase, ending at 0.3% attB and 7% attP. This decline in spontaneous induction frequency is consistent with early studies of phage λ that showed that the lysogenic response is favored if the cells are grown past the exponential growth phase (14). The decline in the spontaneous induction rate and the level of extrachromosomal phage DNA was not seen in a 24-h-old culture grown at 15°C (Fig. (Fig.2B).2B). This was probably due to the fact that the slow-growing culture at 15°C did not enter the stationary phase before 18 to 20 h and was still in its early stationary phase after 24 h. Consequently, the overnight culture used as an inoculum consisted of cells with a relatively high spontaneous induction frequency. Reinitiation of cell growth in fresh medium led to the expected degradation of the induced cells, seen as a decrease in the levels of the attP and attB DNA sites (Fig. (Fig.2B)2B) due to phage burst and lysis of the induced stationary-phase cells.

During growth, the level of the attL DNA sites was not significantly different from the level of the chr DNA sites as shown by the statistical tests described above, confirming that most of the L. lactis IMN-C1814 cells remained lysogenic. These findings were consistent with the calculated prophage induction rates of between 0.4 and 1.3% at 30°C and between 0.4 and 1.6% at 15°C, depending on the growth stage of the lysogenic bacterium.

The level of the attP DNA sites reflects the level of extrachromosomal [var phi]LC3 phage DNA in a population of cells, in which the phages may be in different stages in their development due to different time points of induction. Hence, the ratio between the [var phi]LC3 attP and attB DNA sites gives an estimate of the average amount of phage genomes in the induced cells, and consequently the attP/attB ratio will indirectly reflect the burst size. In the stationary phase during growth of IMN-C1814 (Fig. (Fig.2),2), the attP/attB ratio remained at approximately 30 at both 15 and 30°C. This attP/attB ratio agrees with a burst size of about 60 phages (26). Furthermore, these results indicated that the phage DNA propagation efficiency seemed to be unaffected by temperature.

In the stationary phase during growth of IMN-C1814 (Fig. (Fig.2)2) the number of [var phi]LC3 phages at 30°C was about 106 to 107 ml−1, while the culture grown at 15°C contained 10- to 100-fold fewer PFU. These levels of free phage particles are consistent with the levels reported for other temperate lactococcal phages in overnight cultures of lysogenic strains (8, 38). A reduction in the number of infectious phage particles in cultures with lower growth rates was also observed in studies on lambdoid prophages in E. coli cultures (9). However, Czyz et al. (9) could not determine if the reduction was caused either by a lower frequency of prophage induction or by less efficient lytic development after excision of the phage from the host chromosome.

From the results obtained in this study, it can be concluded that the lower number of infectious [var phi]LC3 phages in cultures grown at 15°C compared to 30°C was obviously not due to a lower frequency of prophage induction in the slowly growing culture (Fig. (Fig.2)2) or to a less efficient propagation of excised phage DNA (see above). It seemed more likely that the maturation process of phage particles, including packaging, was less efficient at low temperature. Comparative results have been obtained for the E. coli bacteriophage T4, in which the burst size of the phage is not limited by cell size or DNA composition, or directly by the rate of metabolism, but rather is limited by the rate of synthesis and assembly of phage components and by lysis time (16).

[var phi]LC3 prophage stability is dependent on host strain and growth temperature.

The decision of the temperate phage to enter either a lytic or lysogenic life cycle is believed to be affected by phage- and host-encoded factors in addition to environmental conditions (37). Studies regarding this lysis-lysogeny decision of temperate phages have been hampered by the lack of good techniques. In this work we have analyzed the spontaneous rate of induction of [var phi]LC3 in six lysogenic strains when grown at 15 and 30°C by the new method of Lunde et al. (29).

The strains used in this study were the parental strain of [var phi]LC3, IMN-C3, and five additional constructed [var phi]LC3 lysogenic lactococcal strains (Table (Table1).1). The new lysogenic derivates were still inducible and tested to be resistant to superinfection by [var phi]LC3 after more than 10 rounds of single-colony isolations. The results strongly suggested that the strains were true [var phi]LC3 lysogens. Furthermore, the use of sequence specific primers for amplification of the phage [var phi]LC3 regulator gene, orf286 (3), verified that the strains in use were [var phi]LC3 lysogenic at the time point of analysis. The presence of TP936-1 in the double lysogenic strain 3107::TP936-1::[var phi]LC3 was verified by PCR with primers specific to the TP901-1 repressor gene (orf4) (30) (results not shown).

The previous experiments with the lysogenic strain IMN-C1814 (Fig. (Fig.2)2) showed that the highest frequency of [var phi]LC3 prophage induction was obtained when the cells were about to enter the stationary phase. This result indicated that the induction seemed to depend on the cell growth stage. Consequently, it was important to obtain the growth kinetics of the six lysogenic strains at both growth temperatures (15 and 30°C) in order to compare the spontaneous induction frequency at the same growth stage when the prophage was most unstable. At 30°C the lysogenic cultures entered the stationary phase after approximately 6 to 8 h, while at 15°C they entered the stationary phase after approximately 19 to 23 h, depending on the individual strain. The generation times for the six strains at 30 and 15°C, respectively, were as follows: C1814, 1.08 and 2.2 h; C3, 1.05 and 2.8 h; C17::[var phi]LC3, 0.99 and 2.95 h; C20::[var phi]LC3, 1.26 and 2.65 h; 3107::[var phi]LC3, 0.73 and 3.89 h; and 3107::TP901-1::[var phi]LC3, 1.03 and 3.99 h. DNA samples were isolated from the cultures at three time points, one at the end of the exponential growth phase and two in the early stationary phase. The average ratio of attB to chr at these three time points was used to determine the [var phi]LC3 prophage stability of each strain, and the results are presented and discussed below.

The strains showed considerable variability in prophage stability, as shown in Fig. Fig.3A.3A. Between the lowest (IMN-C17::[var phi]LC3 at 15°C, 0.32% attB) and the highest (IMN-C20::[var phi]LC3 at 15°C, 9.1% attB) spontaneous prophage induction frequency, a 28-fold difference was observed. Growth at high and low temperatures did not affect the [var phi]LC3 prophage stability in the lysogenic hosts in one direction (Fig. (Fig.3A).3A). The spontaneous induction rates were higher at 30°C than at 15°C for the three strains IMN-C3, 3107::[var phi]LC3, and 3107::TP936-1::[var phi]LC3 (1.7-, 2.9-,and 1.8-fold, respectively), while the two strains IMN-C1814 and IMN-C20::[var phi]LC3 showed the highest spontaneous induction rates at 15°C (1.2-and 2.1-fold, respectively). Growth at two different temperatures did not significantly alter the induction rate in IMN-C17::[var phi]LC3 even though the generation times varied from 1 to 2.9 h at 15 and 30°C, respectively. Compared to the stability in the single lysogenic 3107::[var phi]LC3 strain, the [var phi]LC3 prophage was more stable in the double lysogenic strain 3107::TP936-1::[var phi]LC3 at 30°C, while no significant difference in prophage stability was seen at 15°C. The fact that the growth rate of the double lysogenic strain was lower than that of the single lysogenic strain at 30°C but did not differ at 15°C may explain the lower induction rate of 3107::TP936-1::[var phi]LC3 at 30°C. However, the IMN-C1814, IMN-C3, IMN-C17::[var phi]LC3, and 3107::TP936-1::[var phi]LC3 strains showed approximately the same generation time of 1 h at 30°C, but their prophage induction frequencies varied between 1 and 6.3%. From the results obtained from this study, it can be concluded that the different growth rates alone certainly cannot explain the differences in prophage stability between the different strains and that changes in growth temperature affect the induction rates in different directions.

FIG. 3.
Spontaneous induction of the [var phi]LC3 prophage into the lytic life cycle and the following phage DNA propagation in the Lactococcus lactis subsp. cremoris strains IMN-C1814 (bars a), IMN-C3 (bars b), IMN-C17::[var phi]LC3 (bars c), IMN-C20::[var phi]LC3 ...

The different spontaneous induction rates of [var phi]LC3 in the single and double lysogenic derivatives of the 3107 strain at 30°C may indicate that some kind of cross-communication between the two P335 phages (20) may play a role in [var phi]LC3 prophage stability; however, this has to be investigated further. Furthermore, DNA from phage [var phi]LC3 hybridized to genomic DNA obtained from the lactococcal [var phi]LC3 host strains, except for IMN-C18 (Table (Table1),1), before they were lysogenized with [var phi]LC3 (data not shown). Hence, it cannot be ruled out that putative phage factors other than from [var phi]LC3 played a role in the regulation of the lysis-lysogeny decision of [var phi]LC3. However, the attB and attP DNA sites detected by real-time PCR in this study were specific to [var phi]LC3, since the level of the attB sites in the non-[var phi]LC3 lysogenic strains was equivalent to the amount of cells (chr marker) in DNA samples obtained from the cultures. Hence, no other phage occupied the attachment site of phage [var phi]LC3. Furthermore, no attP DNA was detected in the host strains before they were lysogenized with [var phi]LC3 (results not shown).

[var phi]LC3 DNA propagation is unique for each host strain and depends on the growth temperature.

After prophage induction, the phage follows the lytic pathway, which involves DNA replication, expression of phage-encoded genes, packaging, maturation, and lysis of the host cell (37). We have studied the effect of growth temperature on the [var phi]LC3 phage DNA propagation after induction of the phage from its lysogenic to its lytic state in six different hosts by determination of the ratio between the attP and attB DNA sites in genomic DNA samples.

As stated above, the attP/attB ratio gives an indirect estimate of the burst size of [var phi]LC3 in the induced cells, and the results shown in Fig. Fig.3C3C indicated that the theoretical burst sizes of the six lysogenic strains varied between 16 and 120 phage particles. These burst sizes are similar to values documented in the literature for other lactococcal phages (21). However, to our knowledge this is the first study that shows that the burst size of one particular temperate phage varies between different hosts (up to eightfold) after being induced from the lysogenic to the lytic pathway. The formation of the attB and attP DNA sites in the lysogenic strains was generally within expected values, since when large amounts of attB sites were formed (Fig. (Fig.3A),3A), correspondingly high levels of attP sites were produced (Fig. (Fig.3B).3B). However, the quantitative relationship between attB and attP was not consistent between the strains and between temperatures, as shown in Fig. Fig.3C.3C. As described above, the attP/attB ratio in IMN-C1814 was approximately 30 at both 15 and 30°C, and this result indicated that the phage DNA propagation in the lytic state was not affected by growth temperature. For the three strains IMN-C3, IMN-C17::[var phi]LC3, and IMN-C20::[var phi]LC3, propagation of the [var phi]LC3 DNA genome seemed to be more efficient at 30 than at 15°C, since the attP/attB ratio was approximately twofold higher at the highest temperature. In contrast, for both the single and double lysogenic strains of 3107, the attP/attB ratio was about 2.5-fold higher at 15 than at 30°C, which indicated that the phage DNA propagation was more efficient at the lower temperature for these two lysogenic strains. Furthermore, the attP/attB ratio was 1.7-fold higher in the single lysogenic strain than in the double lysogenic strain of 3107 at both temperatures. The results obtained here indicated that the propagation of [var phi]LC3 DNA after induction of the prophage into the lytic life style differs between different hosts and that growth temperature affects the propagation step differently in the host strains.

Conclusions.

The temperate lactococcal phage [var phi]LC3 originally induced from the industrial strain L. lactis subsp. cremoris IMN-C3 was able to exist as a lysogen in several hosts. The results from this study showed that the spontaneous frequency of induction of the [var phi]LC3 prophage was dependent on the growth phase of the host cell. Furthermore, the frequency of spontaneous prophage induction and the estimated burst sizes varied considerably (28- and 8-fold, respectively) between the lysogenic hosts regardless of the growth rate. Changes in growth temperature did not lead to a uniform response with regard to the spontaneous prophage induction and extrachromosomal phage DNA propagation in the lytic pathway. The present results add to the well-established knowledge that the genetic switch between the lytic and lysogenic pathways is subjected to complex regulation and that the host contributes significantly to prophage stability.

We believe that this work, with the use of phage [var phi]LC3 and its host as a model system, has provided important knowledge about the complex interaction between a temperate phage and its host. Currently we are exploring how dairy-related environmental stress, such as variations in temperature, pH, osmolarity, and nutritional resources, influences both the prophage stability and the lysogenization process of [var phi]LC3. Such information will be useful to reveal the impact of lysogeny in bacterial fermentation processes as well as in other ecological systems. This study also shows the innovative use of a new technology in the analysis of temperate-phage biology. We believe that this method is also a valuable tool for studying the effects of various phage- and host-encoded gene products on phage development and phage-host interactions.

Acknowledgments

We thank Linda Godager for skilled technical assistance and Elizabeth Andersen Rawlinson for helpful discussions regarding real-time quantitative PCR.

This work was supported by grants from the European Union Biotechnology program (contract BIO-CT96-0402) and the Norwegian Research Council (project no. 140438/130).

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