Lata Venkataraman

Linezolid resistance in a clinical isolate of Staphylococcus aureus.

The new oxazolidinone antimicrobial, linezolid, has been approved for the treatment of infections caused by various gram-positive bacteria, including meticillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). Although instances of linezolid resistance in VRE have been reported, resistance has not been encountered among clinical isolates of S aureus. We have characterised an MRSA isolate resistant to linezolid that was recovered from a patient treated with this agent for dialysis-associated peritonitis.

Accessory Gene Regulator (agr) Locus in Geographically Diverse Staphylococcus aureus Isolates with Reduced Susceptibility to Vancomycin

ABSTRACT The majority of infections with glycopeptide intermediate-level resistant Staphylococcus aureus (GISA) originate in biomedical devices, suggesting a possible increased ability of these strains to produce biofilm. Loss of function of the accessory gene regulator (agr) of S. aureus has been suggested to confer an enhanced ability to bind to polystyrene. We studied agr in GISA, hetero-GISA, and related glycopeptide-susceptible S. aureus isolates. All GISA strains from diverse geographic origins belong to agr group II. All GISA strains were defective in agr function, as demonstrated by their inability to produce delta-hemolysin. Hetero-GISA isolate A5940 demonstrated a nonsense mutation in agrA that was not present in a pulsed-field gel electrophoresis-indistinguishable vancomycin-susceptible isolate from the same patient. Various other agr point mutations were noted in several clinical GISA and hetero-GISA isolates. A laboratory-generated agr-null strain demonstrated a small but reproducible increase in vancomycin heteroresistance after growth in vitro in subinhibitory concentrations of vancomycin. This was not seen in the isogenic agr group II parent strain in which agr was intact. The in vitro bactericidal activity of vancomycin was attenuated in the agr-null strain compared to the parent strain. These findings imply that compromised agr function is advantageous to clinical isolates of S. aureus toward the development of vancomycin heteroresistance, perhaps through the development of vancomycin tolerance.

Clinical and molecular epidemiology of sporadic and clustered cases of nosocomial Clostridium difficile diarrhea

PURPOSE A prospective clinical and molecular epidemiologic study was conducted to define the frequency of nosocomial Clostridium difficile patient-to-patient transmission in an urban tertiary referral hospital. PATIENTS AND METHODS Over a 6-month period, environmental cultures for C difficile were obtained from patients with new positive stool cytotoxin assay (index cases); stool samples were obtained from selected patient contacts (the roommate, occupants of adjacent rooms, and the patient occupying the index room after discharge of the index case); and hand cultures were obtained from personnel contacts. C difficile isolates were analyzed by pulse-field gel electrophoresis (PFGE) or, for isolates that were nontypeable by PFGE, by restriction enzyme analysis. RESULTS During the study period, we identified 98 index cases of C difficile toxin-associated diarrhea, including focal outbreaks on two wards totaling 26 cases within a 2-month interval. Environmental contamination was detected at > or = 1 sites in 58% of rooms and often involved wide dispersed areas. Among 99 prospectively identified patient contacts, C difficile was cultured from the stool of 31 (31%), including 12 with diarrhea and 19 who were asymptomatic. C difficile was cultured from the hands of 10 (14%) of 73 personnel. Molecular analysis resolved 31 typing profiles among the index isolates; the most common profile (designated strain D1) was represented by 30 isolates. Among the isolates from patient contacts, 5 of 12 from symptomatic contacts matched the corresponding index isolate, and only 1 of 19 from asymptomatically colonized contacts matched. Transmission to personnel or patient contacts of the strain cultured from the corresponding index case was correlated strongly with the intensity of environmental contamination. Strain D1 was frequently represented among isolates associated with heavy environmental contamination, with personnel carriage, and with development of symptomatic illness among prospectively identified contacts. CONCLUSIONS Intense environmental contamination and transmission to close personnel and patient contacts represented coordinated properties of an individual epidemic strain. For most epidemiologically linked contacts, positive cultures for C difficile did not result from transmission from the presumed index case.

Linezolid Resistance in Sequential Staphylococcus aureus Isolates Associated with a T2500A Mutation in the 23S rRNA Gene and Loss of a Single Copy of rRNA

Linezolid is an important therapeutic option for infections caused by resistant gram-positive bacteria. We report the characterization of sequential methicillin-resistant Staphylococcus aureus (MRSA) bloodstream isolates that developed resistance in a patient treated with a prolonged course of linezolid. Analysis of this series of clinical MRSA isolates detected, in the resistant isolates, the presence of a T2500A mutation in the domain V region of the 23S rRNA gene. In addition, the loss of a single copy of the 23S rRNA gene was found in 2 of the resistant isolates. As a result of these 2 factors, the proportion of mutant : wild-type 23S rRNA genes increased in association with an increase in the minimum inhibitory concentration of linezolid. The most recent isolate of this series was recovered 7 months after the patient discontinued linezolid and demonstrated reversion to a susceptible phenotype associated with a loss of the T2500A mutation.

Staphylococcus aureus Accessory Gene Regulator (agr) Group II: Is There a Relationship to the Development of Intermediate-Level Glycopeptide Resistance?

We previously determined that all 6 Staphylococcus aureus strains with confirmed intermediate-level resistance to glycopeptides (glycopeptide intermediate S. aureus [GISA]) from the United States that we tested belonged to accessory gene regulator (agr) group II. In the present study, we found that 56% of surveyed bloodstream methicillin-resistant S. aureus isolates (n = 148) at our hospital were agr group II, whereas only 24% of methicillin-susceptible S. aureus isolates (n = 33) were agr group II (P = .001). Population analysis of genetically engineered agr-null and parent wild-type strains of groups I, II, and IV revealed that, when agr function is lost, the agr group II knockout S. aureus was most likely to develop glycopeptide heteroresistance after growth in 1 microg/mL but not 16 microg/mL vancomycin. This strain was unique in showing decreased autolysis after growth in these conditions. This study suggests that some S. aureus strains have an intrinsic survival advantage under a glycopeptide selective pressure, which is possibly related to reduced autolysis after exposure to subinhibitory concentrations of glycopeptide.

Methicillin-Resistant Staphylococcus aureus: Comparison of Susceptibility Testing Methods and Analysis of mecA-Positive Susceptible Strains

ABSTRACT Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for an increasing number of serious nosocomial and community-acquired infections. Phenotypic heterogeneous drug resistance (heteroresistance) to antistaphylococcal beta-lactams affects the results of susceptibility testing. The present study compared the MRSA-Screen latex agglutination test (Denka Seiken Co., Ltd., Tokyo, Japan) for detection of PBP 2a with agar dilution, the VITEK-1 and VITEK-2 systems (bioMérieux, St. Louis, Mo.), and the oxacillin agar screen test for detection of MRSA, with PCR for themecA gene used as the “gold standard” assay. Analysis of 107 methicillin-susceptible S. aureus (MSSA) isolates and 203 MRSA isolates revealed that the MRSA-Screen latex agglutination test is superior to any single phenotype-based susceptibility testing method, with a sensitivity of 100% and a specificity of 99.1%. Only one isolate that lacked mecAwas weakly positive by the MRSA-Screen latex agglutination test. This isolate was phenotypically susceptible to oxacillin and did not contain the mecA gene by Southern blot hybridization. The oxacillin agar screen test, the VITEK-1 system, the VITEK-2 system, and agar dilution showed sensitivities of 99.0, 99.0, 99.5, and 99%, respectively, and specificities of 98.1, 100, 97.2, and 100%, respectively. The differences in sensitivity or specificity were not statistically significant. Oxacillin bactericidal assays showed thatmecA- and PBP 2a-positive S. aureusisolates that are susceptible to antistaphylococcal beta-lactams by conventional methods are functionally resistant to oxacillin. We conclude that the accuracy of the MRSA-Screen latex agglutination method for detection of PBP 2a approaches the accuracy of PCR and is more accurate than any susceptibility testing method used alone for the detection of MRSA.

Epidemiology and Clinical Outcomes of Patients with Multiresistant Pseudomonas aeruginosa

We conducted a case-series study of multiresistant Pseudomonas aeruginosa in patients who did not have cystic fibrosis. Patient characteristics, antibiotic exposures, time course of emergence of resistance, and clinical outcomes were examined. Twenty-two patients were identified from whom P. aeruginosa resistant to ciprofloxacin, imipenem, ceftazidime, and piperacillin was isolated. Nineteen (86%) had clinical infection. Patients received prolonged courses of antipseudomonal antibiotics before isolation of multiresistant P. aeruginosa. Nine of 11 patients with soft-tissue infection exhibited resolution of clinical infection but usually required surgical removal of infected tissue with or without revascularization. Overall, three patients died. In two instances in which multiple isolates with different susceptibility profiles from the same patient were available, pulsed-field gel electrophoresis profiles of serial isolates were indistinguishable or closely related. This study illustrates that multiresistant P. aeruginosa emerges in a stepwise manner after exposure to antipseudomonal antibiotics and results in adverse outcomes.

Emergence of a Clinical Daptomycin-Resistant Staphylococcus aureus Isolate during Treatment of Methicillin-Resistant Staphylococcus aureus Bacteremia and Osteomyelitis

ABSTRACT The emergence of a clinically daptomycin-resistant Staphylococcus aureus isolate occurred during treatment of methicillin-resistant S. aureus bacteremia and probable vertebral osteomyelitis. The breakthrough isolate was indistinguishable from pretreatment daptomycin-susceptible isolates by pulsed-field gel electrophoresis. Daptomycin nonsusceptibility was confirmed by MIC and time-kill curve analyses.

Linezolid-Resistant, Vancomycin-Resistant Enterococcus faecium Infection in Patients without Prior Exposure to Linezolid

We describe 2 patients without prior exposure to linezolid who were infected with closely related strains of linezolid- and vancomycin-resistant Enterococcus faecium (LRVREF) that may have been hospital acquired. Polymerase chain reaction amplification of the domain V region of the 23S ribosomal RNA gene demonstrated the presence of the G2576U mutation previously reported to be associated with linezolid resistance. Nosocomial transmission of LRVREF is an ominous sign and underscores the importance of meticulous infection-control measures.

Colonization with broad-spectrum cephalosporin-resistant gram-negative bacilli in intensive care units during a nonoutbreak period: prevalence, risk factors, and rate of infection.

OBJECTIVE To define the epidemiology of broad-spectrum cephalosporin-resistant gram-negative bacilli in intensive care units (ICUs) during a nonoutbreak period, including the prevalence, the risk factors for colonization, the frequency of acquisition, and the rate of infection. DESIGN Prospective cohort study. SETTING Tertiary care hospital. PATIENTS Consecutive patients admitted to two surgical ICUs. MAIN OUTCOME MEASUREMENTS Serial patient surveillance cultures screened for ceftazidime (CAZ) resistance, antibiotic and hospital exposure, and infections. RESULTS Of the 333 patients enrolled, 60 (18%) were colonized with CAZ-resistant gram-negative bacilli (CAZ-RGN) at admission. Clinical cultures detected CAZ-RGN in only 5% (3/60) of these patients. By using logistic regression, CAZ-RGN colonization was associated with duration of exposure to cefazolin (odds ratio, 10.3; p < or = .006) and to broad-spectrum cephalosporins/penicillins (odds ratio, 2; p < or = .03), Acute Physiology and Chronic Health Evaluation III score (odds ratio, 1.2; p < or = .008), and previous hospitalization (odds ratio, 3.1; p < or = .006). Of the 100 patients who remained in the surgical ICU for > or = 3 days, 26% acquired a CAZ-RGN. Of the 14 infections caused by CAZ-RGN, 11 (79%) were attributable to the same species present in surveillance cultures at admission to the surgical ICU. CONCLUSIONS Colonization with CAZ-RGN was common and was usually not recognized by clinical cultures. Most patients colonized or infected with CAZ-RGN had positive surveillance cultures at the time of admission to the surgical ICU, suggesting that acquisition frequently occurred in other wards and institutions. Patients exposed to first-generation cephalosporins, as well as broad-spectrum cephalosporins/penicillins, were at high risk of colonization with CAZ-RGN. Empirical treatment of nosocomial gram-negative infections with broad-spectrum cephalosporins, especially in the critically ill patient, should be reconsidered.