In a study of 40 methicillin-resistant Staphylococcus aureus (MRSA) carriers, hand contamination was equally likely after contact with commonly examined skin sites and commonly touched environmental surfaces in patient rooms (40% vs 45%). These findings suggest that contaminated surfaces may be an important source of MRSA transmission.

We used a mouse model to test the hypothesis that anaerobic microbiota in the colon inhibit the establishment of vancomycin-resistant enterococci (VRE) colonization by depleting nutrients within cecal contents and limiting the association of VRE with the mucus layer. Anaerobic growth of VRE was assessed in cecal contents and cecal mucus of mice that had received treatment with subcutaneous clindamycin or saline. VRE grew to high concentrations in cecal contents of clindamycin-treated mice and in cecal mucus of both groups but not in cecal contents of saline-treated mice, unless the cecal contents were autoclaved or converted into sterile filtrates. After orogastric inoculation of VRE, clindamycin-treated mice acquired high concentrations of VRE within the mucus layer, whereas saline-treated mice did not. These results suggest that colonic microbiota inhibit VRE by producing inhibitory substances or conditions rather than by depleting nutrients. The colonic mucus layer provides a potential niche for growth of VRE.

Patients receiving anti-anaerobic antibiotics, including piperacillin/tazobactam, may be susceptible to the establishment of high-density VRE colonization during the period of recovery of the anaerobic microflora.

Before any contact precautions can be implemented, newly identified MRSA carriers frequently have contaminated their environment with MRSA and have contamination of commonly examined skin sites. In hospitals that perform active surveillance, strategies are needed to minimize delays in screening or to preemptively identify patients at high risk for disseminating MRSA.

In a prospective study involving 115 patients with methicillin-resistant Staphylococcus aureus nares carriage, we found that skin and environmental contamination with methicillin-resistant S. aureus was as likely among individuals whose S. aureus carriage was identified only through active surveillance as it was among those individuals whose S. aureus carriage was identified clinically, which suggests that strategies to limit transmission must address colonized patients, as well as infected patients.

In mice, vancomycin and linezolid treatment disrupted the anaerobic intestinal microflora, based on denaturing gradient gel electrophoresis analysis, and promoted colonization by Klebsiella pneumoniae and vancomycin-resistant enterococci. However, the effects varied depending on dose and duration of treatment. Daptomycin treatment did not disrupt the anaerobic microflora or promote either pathogen.In case-control studies, exposure to vancomycin has been associated with colonization and infection due to vancomycinresistant enterococci (VRE) (8). Oral or parenteral vancomycin administration has also been shown to promote persistent overgrowth of VRE in the stool of colonized mice (3). It is not known whether other agents that are available for treatment of methicillin-resistant gram-positive pathogens (e.g., daptomycin, linezolid) also promote VRE colonization. Because linezolid and daptomycin possess inhibitory activity against most VRE strains (1, 5), we used a mouse model to test the hypothesis that these agents promote establishment of VRE colonization to a lesser degree than vancomycin. We also examined the effect of the antibiotics on establishment of colonization by extended-spectrum ␤-lactamase (ESBL)-producing Klebsiella pneumoniae, a pathogen with intrinsic resistance to each of the antibiotics.The VRE strain used in this study (C68) is a clinical VanB isolate that we have used for previous mouse colonization studies (3). The MICs for C68 were 256 g/ml with vancomycin, 0.25 g/ml with daptomycin, and 1 g/ml with linezolid. K. pneumoniae strain P62 produces an SHV ESBL and has been used in previous mouse studies (9).Two doses of the antibiotics were studied, as follows: 1 or 12.5 mg/day for vancomycin, 0.6 or 7.5 mg/day for linezolid, and 0.12 or 1.5 mg/day for daptomycin. The lower dose of antibiotic was based on the daily dose recommended for human adults (in milligrams per kilogram of body weight), and the higher dose was the human equivalent dose calculated by the technique of Freireich et al. (6). The daptomycin, vancomycin, and linezolid doses were calculated based on 6, 33.3, and 20 mg/kg human doses, respectively. All antibiotics were administered subcutaneously once each day in 0.2 ml of phosphate-buffered saline, with the exception of the higher dose of linezolid, which was administered by orogastric gavage once each day because the formulations available were not conducive to subcutaneous dosing (i.e., the concentration of the intravenous formulation would have resulted in an inordinately high volume for subcutaneous administration).

We examined the effect of antibiotic treatment on establishment of intestinal colonization by Candida glabrata in adult mice. Subcutaneous ceftriaxone, piperacillin-tazobactam, clindamycin, and metronidazole promoted increased density of stool colonization, whereas cefepime, levofloxacin, and aztreonam did not. These findings suggest that antibiotics that inhibit intestinal anaerobes promote C. glabrata colonization.The gastrointestinal tract is a primary site of colonization by Candida species and is frequently a source for hematogenous dissemination in cancer patients (1). Antibiotics play a crucial role in the pathogenesis of Candida infections by inhibiting competing indigenous microflora, thereby facilitating acquisition and overgrowth of Candida species in the gastrointestinal tract and at other sites (1). Several studies suggest that antibiotics that inhibit obligate anaerobes in the intestinal tract may be more likely to promote overgrowth of C. albicans than those that do not (1,4,5,7,(8)(9)(10)(11), but little data regarding the effect of antibiotics on colonization with other Candida species are available. We used a mouse model to test the hypothesis that antibiotics with potent activity against intestinal anaerobes (piperacillin-tazobactam, metronidazole, ceftriaxone, and clindamycin) promote overgrowth of Candida glabrata, whereas agents with minimal activity against anaerobes (cefepime, levofloxacin, and aztreonam) do not.Four C. glabrata strains were used: ATCC 90030, MRL 191, A129, and A239. The fluconazole MICs for the strains were 32, 16, 0.5, and 4 g/ml, respectively. The experimental protocol was approved by the Cleveland Veterans Affairs Medical Center's Animal Care Committee. Female CF1 mice (Harlan Sprague-Dawley, Indianapolis, Ind.) weighing 25 to 30 g were used. To prevent cross-contamination, mice were housed in individual cages with filter tops. An initial experiment (four mice per group) was performed to examine the ability of the four C. glabrata strains to colonize the intestinal tract. Mice were treated daily with subcutaneous normal saline (0.2 ml) or piperacillin-tazobactam (8 mg in 0.2 ml of normal saline) for 2 days before and 6 days after orogastric inoculation of 10 8 CFU of C. glabrata with a stainless steel feeding tube (Popper & Sons, New Hyde Park, N.Y.). Piperacillin-tazobactam was chosen for the initial experiment because we have previously demonstrated that it inhibits anaerobes, facultative gram-negative bacilli, and enterococci in the stool of mice (2). Fresh stool samples were collected at baseline and 1, 3, and 6 days after inoculation of C. glabrata. For quantification of C. glabrata, serially diluted stool samples were plated onto Sabouraud dextrose agar (Becton, Dickinson, and Company, Sparks, Md.) containing piperacillin-tazobactam (16 g/ml) and incubated in room air at 37°C for 48 h. The lower limit of detection was ϳ2 log 10 CFU/g. Strains A239 and ATCC 90030 were chosen to evaluate the effect of different antibiotics on establishment of colonization because ...

Antibiotics that are excreted into the intestinal tract may disrupt the indigenous intestinal microbiota and promote colonization by health care-associated pathogens. ␤-Lactam, or penicillin-type, antibiotics are among the most widely utilized antibiotics worldwide and may also adversely affect the microbiota. Many bacteria are capable, however, of producing ␤-lactamase enzymes that inactivate ␤-lactam antibiotics. We hypothesized that prior establishment of intestinal colonization with a ␤-lactamaseproducing anaerobe might prevent these adverse effects of ␤-lactam antibiotics, by inactivating the portion of antibiotic that is excreted into the intestinal tract. Here, mice with a previously abolished microbiota received either oral normal saline or an oral cephalosporinase-producing strain of Bacteroides thetaiotaomicron for 3 days. Mice then received 3 days of subcutaneous ceftriaxone, followed by either oral administration of vancomycin-resistant Enterococcus (VRE) or sacrifice and assessment of in vitro growth of epidemic and nonepidemic strains of Clostridium difficile in murine cecal contents. Stool concentrations of VRE and ceftriaxone were measured, cecal levels of C. difficile 24 h after incubation were quantified, and denaturing gradient gel electrophoresis (DGGE) of microbial 16S rRNA genes was performed to evaluate the antibiotic effect on the microbiota. The results demonstrated that establishment of prior colonization with a ␤-lactamase-producing intestinal anaerobe inactivated intraintestinal ceftriaxone during treatment with this antibiotic, allowed recovery of the normal microbiota despite systemic ceftriaxone, and prevented overgrowth with VRE and epidemic and nonepidemic strains of C. difficile in mice. These findings describe a novel probiotic strategy to potentially prevent pathogen colonization in hospitalized patients.