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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Am J Infect Control. Author manuscript; available in PMC May 1, 2011.
Published in final edited form as:
PMCID: PMC2860683
NIHMSID: NIHMS170583

Low Prevalence of Acinetobacter baumannii colonization upon Hospital Admission

Kerri A. Thom, MD, MS,1,* Anthony D. Harris, MD, MPH,1 Judith A. Johnson, PhD,1,2,** and Jon P. Furuno, PhD1

Abstract

We conducted a prospective cohort study of non-critically-ill patients admitted to a tertiary-care center from December 2003 to September 2004 and obtained peri-rectal samples at hospital admission to determine the prevalence of Acinetobacter baumannii. A. baumannii was isolated from 1 (0.18%) of 555 cultures (no multidrug-resistant A. baumannii was identified). Interventions aimed at early identification of A. baumannii colonized patients may not benefit by widely targeting non-critically-ill patients upon hospital admission.

Acinetobacter baumannii is an emerging nosocomial pathogen with important clinical consequences1. Colonization with A. baumannii may precede clinical infection24 and may contribute nosocomial spread. Therefore, knowledge about the epidemiology of A. baumannii colonization is important to identify persons at potential risk of infection and to target infection control interventions aimed at decreasing transmission. Most studies investigating colonization with A. baumannii have focused on critically-ill populations and thus, the prevalence of colonization in non-critically-ill patients is largely unknown‥ In this study, we aimed to determine the prevalence of peri-rectal colonization with A. baumannii among non-critically-ill patients upon hospital admission.

METHODS

Patient data and isolates were collected from a previous prospective cohort of patients admitted to general medical and surgical units of the University of Maryland Medical Center (UMMC) to identify risk factors for presence of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) upon hospital admission (methods described elsewhere)5. Briefly, a sample of patients admitted between December 16th, 2003 and September 9th, 2004 were invited to participate. Patients with known history of MRSA or VRE colonization/infection were excluded. Peri-rectal culture was obtained within 48 hours of hospital admission. Patient data were collected from a questionnaire and from the UMMC central data repository, a relational database containing patient administrative and laboratory data. This study was approved by the University of Maryland Institutional Review Board.

After collection, peri-rectal cultures were stored frozen in Trypticase Soy Broth (TSB) with 30% glycerol (BBL/Becton Dickenson; Sparks,MD). Cultures were later thawed and plated onto MacConkey agar (Remel Inc.; Lenexa,KS) and MacConkey agar supplemented with imipenem (US Pharmacopia; Rockville, MD) and were incubated at 37°C for 24–48 hours. Recovery of organisms from ar chived frozen specimens using this technique has been shown to be 98%6. Organisms were identified as A. baumannii using standard laboratory methods.

RESULTS

During the study period 4710 eligible patients were dmitted to the general medical and surgical wards at UMMC. Of these, 2819 (59.9%) were randomly selected and underwent screening for study participation and 555 (11.8%) were enrolled and underwent peri-rectal surveillance culturing. Differences between participants and non-participants have been previously described5.

The mean (SD) age of the study sample was 53 (16) years and 50% of the study patients were men. 68% (379/555) of patients reported receiving an antibiotic within the previous year and 66% (368/555) reported a prior admission to any healthcare facility within the previous year. During the index admission 60% (332/555) of patients received an antibiotic, the mean (SD) Charlson Co-morbidity Index was 2.1 (2.5), 5% (28/555) of patients were admitted to an intensive care unit, and 0.4% (2/555) of patients died.

A. baumannii was isolated from 1 (0.18%) of the 555 peri-rectal cultures and no isolates of multidrug-resistant A. baumannii (MDR-AB) were identified. One (0.18%) of the 555 patients enrolled had a clinical culture positive for A. baumannii from a thigh wound at the time of hospital admission (i.e. near the time of study enrollment); however, the peri-rectal culture for this patient was negative for growth of A. baumannii. Among the 2819 patients screened, 16 (0.57%) were found to have a clinical culture positive for growth of A. baumannii from multiple sources; in 7 patients cultures were positive within the first 48 hours of admission.

DISCUSSION

Among non-critically-ill patients in this study, less than 1% was colonized in the gastrointestinal tract with A. baumannii and no patients were colonized with MDR- AB. In addition, only one patient among those who underwent peri-rectal culturing was identified as having a clinical culture positive for growth of A. baumannii (and this patient did not have growth of A. baumannii from the peri-rectal culture).

The incidence of hospital-acquired A. baumannii infections, particularly MDR-AB is on the rise1. Colonization with A. baumannii has been shown to precede infection; previous studies showing that 17–26% of patients colonized with A. baumannii at one or multiple sites (e.g. skin, oropharynnx, rectum) may go to develop clinical infection24. Prior knowledge of colonization status has been linked to higher rates of appropriate empiric therapy for gram-negative bacteremia7 which may result in improved patient outcomes8,9. Furthermore, recognition of colonization before infection allows for the implementation of infection control strategies, such as Contact Precautions, aimed at reducing hospital transmission. Identification of colonization prior to infection, however, relies on costly and time consuming active surveillance strategies. The results of this study suggest a very low prevalence of A. baumannii in the non-critically-ill medical-surgical patient population upon hospital admission, suggesting that active surveillance in this population may not be cost effective. A similar study performed in the intensive care setting, which assumes the largest burden of A. baumannii infections, showed prevalence at admission of 0.82% (10 of 1223 admissions).10

This study had several limitations. The original study cohort excluded patients with known MRSA and/or VRE, which could have resulted in underestimating the prevalence of A. baumannii since risk factors, including previous healthcare exposures, may be similar to those for A. baumannii. However, the prevalence of MRSA and VRE on admission was 7% and 5%, respectively, suggesting a high prevalence despite those exclusions. However, only the peri-rectal site was evaluated in this study and thus we may still have underestimated the prevalence of A. baumannii colonization as other sites such as the skin, wounds, and the respiratory tract may also be colonized4, 11. Finally, since this study was performed from 2003 to 2004 and A. baumannii is continuing to emerge in certain populations, these results may not be consistent with the current epidemiology. A review of UMMC clinical culture data from 2002 to 2008 suggested that the prevalence of A. baumannii did not change substantially in the years before, during or after the study period (Data not shown).

The data presented suggest that the prevalence of gastrointestinal colonization as detected by peri-rectal culture with A. baumannii upon hospital admission in a non-critically-ill population was low. Infection control interventions aimed at early identification of patients colonized with A. baumannii may not benefit by widely targeting non-critically-ill patients upon hospital admission.

Footnotes

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References

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