Evaluation of the CLSI cefoxitin 30-microg disk-diffusion method for detecting methicillin resistance in staphylococci

Wild birds as potential bioindicators of environmental antimicrobial resistance: A preliminary investigation

Antimicrobial resistance (AMR) is an issue of global concern that includes human, animal, and environmental health. To tackle this phenomenon, a One Health approach is required through the involvement of all these interconnected elements. The environment poses challenges for investigation, but wildlife animals, not directly exposed to antibiotic treatments and interacting with their habitats, can serve as indicators of AMR contamination. Specifically, wild birds could play a significant role in dissemination of AMR, as they can acquire AMR bacteria from wildlife reservoirs and disperse them through environments. This study aims to assess the prevalence of AMR in commensal bacteria isolated from wild birds and their role as bioindicators of environmental AMR. A total of 73 birds belonging to various species were sampled in the Emilia-Romagna region with buccal, cloacal and feather samplings. The samples were cultured on selective media, colonies were identified using MALDI-TOF technology and antimicrobial susceptibility to different drugs was assessed using the Kirby-Bauer method. The birds' data were statistically evaluated in relation to AMR percentages. In total, 117 bacterial strains were isolated, belonging to 23 genera and 46 different bacterial species. The highest non-susceptibility percentages were observed for tetracycline (12.2 %) and enrofloxacin (8.6 %) considering all bacterial isolates, as well as for oxacillin (46.8 %), clindamycin (29.3 %) and rifampicin (20.8 %), among Gram-positive isolates. In the statistical analysis, a higher AMR percentage was correlated with Gram-positive isolates from birds belonging to rural/urban habitat (p = 0.01). Among Gram-positives, a higher oxacillin non-susceptibility percentage was found to be associated with isolates from birds sampled in province of Bologna (p = 0.007), a higher enrofloxacin non-susceptibility percentage revealed an association with rural/urban habitat (p = 0.02), while a higher non-susceptibility percentage towards rifampicin resulted associated with isolates from migratory birds (p = 0.031). In conclusion, this preliminary study suggests a potential role of wild birds as bioindicators for monitoring AMR contamination in the environment.

Culex quinquefasciatus Mosquitoes Resist Acquisition of Methicillin-Resistant Staphylococcus aureus: Insights from Field and Laboratory Studies

The emergence of antimicrobial resistance (AMR) in pathogens and their spillover into the environment have become a global public health menace. Insects can acquire these pathogens from the environment and would serve as mechanical and biological vectors. The current study assessed the ability of Culex quinquefasciatus mosquitoes to acquire methicillin-resistant Staphylococcus aureus (MRSA) through the exposure of the mosquitoes to the pathogen via rearing water, blood feed, or a feeding membrane under laboratory conditions. In addition, mosquito immatures collected from their habitat in the vicinity of hospitals, veterinary dispensaries, and butcher shops at 15 study sites in Puducherry were screened for MRSA infection. All samples were subjected to the culture-based isolation of S. aureus from the surface and homogenate. The presence of the S. aureus-specific nuc gene and the genes that confer resistance to methicillin (mecA and mecC) were screened using PCR tests. MRSA was not evident either on the external surface or in the homogenate of the mosquitoes emerging from the larvae reared in the MRSA-spiked water or those fed with MRSA through blood or smeared membranes. Furthermore, the presence of MRSA was not evident in any of the field-caught mosquitoes. Hence, we conclude that C. quinquefasciatus mosquitoes are impervious to MRSA colonization.

[Oxacillin-resistant coagulase-negative Staphylococci bacteremia at a teaching hospital in Santa Maria, State of Rio Grande do Sul, Brazil]

INTRODUCTION

This study aimed to characterize the prevalence and susceptibility profile to oxacillin-resistant Coagulase-negative Staphylococci strains isolated from blood cultures in a teaching hospital, located in Santa Maria, RS. In addition, different methodologies for phenotypic characterization of mecA-mediated oxacillin resistance were compared with genotypic reference testing.

METHODS

After identification (MicroScan - Siemens), the isolates were tested for antimicrobial sensitivity using disk diffusion and automation (MicroScan - Siemens). The presence of mecA gene was identified by the polymerase chain reaction molecular technique.

RESULTS

The most common species was Staphylococcus epidermidis (n=40, 67%). The mecA gene was detected in 54 (90%) strains, while analysis of the sensitivity profiles revealed a high rate of resistance to multiple classes of antimicrobial drugs. However, all isolates were uniformly sensitive to vancomycin and tigecycline. The cefoxitin disk was the phenotypic method that best correlated with the gold standard.

CONCLUSIONS

Analysis of the clinical significance of CoNS isolated from hemocultures and the precise detection of oxacillin resistance represent decisive factors for the correct choice of antibiotic therapy. Although vancomycin constitutes the normal treatment in most Brazilian hospitals, reduction in its use is recommended.

[Evaluation of methods commonly used in laboratories to determine the susceptibility to oxacillin among Staphylococcus sp samples isolated from a hospital in Vitória, State of Espírito Santo]

INTRODUCTION

The genus Staphylococcus is of great importance because of its high prevalence in hospital infections and because it presents high rates of resistance to oxacillin and other antimicrobials. Thus, evaluation of the accuracy of the phenotypic methods that are used to determine the profile of antimicrobial resistance is essential to ensure that the most appropriate therapy is chosen.

METHODS

One hundred and fourteen strains of Staphylococcus sp (53 S. aureus and 61 CNS) were used to evaluate the accuracy of the methods of disk diffusion, agar microdilution, oxacillin screening agar and automated systems, in comparison with PCR for investigating resistance to oxacillin.

RESULTS

The mecA gene was detected in 48 strains (42.1%), and 27 strains (23.7%) showed discrepant results in at least one of the methods (74.1% of CNS, 25.9% of S. aureus). For S. aureus, with the exception of the Microscan Walkaway, all the methods showed 100% specificity and sensitivity. In relation to CNS, the automated system and cefoxitin disk had lower accuracy.

CONCLUSIONS

Use of two methods should be the best option for improved accuracy, especially when the diagnostic laboratory only uses an automated system or oxacillin disk diffusion test. Combination of these methods with others presented almost 100% sensitivity and specificity in our study.

Use of a DNA microarray for simultaneous detection of antibiotic resistance genes among staphylococcal clinical isolates

We developed a multiplex asymmetric PCR (MAPCR)-based DNA microarray assay for characterization of the clinically relevant antibiotic resistance genes leading to penicillin, methicillin, aminoglycoside, macrolide, lincosamide, and streptogramin B (MLS(B)) resistance in staphylococci. The DNA-based assay involves detection of specific conserved regions of the mecA, blaZ (methicillin and penicillin resistance), aac(6')-Ie-aph(2'') (aminoglycoside resistance), ermA and ermC genes (MLS(B) resistance), and the msrA gene (macrolide and streptogramin B resistance). The microarray uses a variable sequence region of the 16S rRNA gene to broadly differentiate between Staphylococcus aureus and other coagulase-negative staphylococci (CoNS). The performance of the microarray was validated with a total of 178 clinically important S. aureus and 237 CoNS isolates, with correlations of 100% for S. aureus to CoNS discrimination and more than 90% for antibiotic resistance between the genotypic analysis determined by the microarray and the phenotype determined by standard methods of species identification and susceptibility testing. The major discrepant results were 17 mecA-positive CoNS and 60 aac(6')-Ie-aph(2'')-positive CoNS isolates measured by microarray that were susceptible to the corresponding antibiotics based on disk diffusion assay. Overall, this microarray-based assay offers a simultaneous, fast (< or =5 h), and accurate identification of antibiotic resistance genes from a single colony, as well as species classification. Our extensive validation of the microarray suggests that it may be a useful tool to complement phenotypic susceptibility testing in clinical laboratories and to survey the spread of antibiotic resistance determinants in epidemiological studies.

Activity of dalbavancin tested against Staphylococcus spp. and beta-hemolytic Streptococcus spp. isolated from 52 geographically diverse medical centers in the United States

Dalbavancin is a lipoglycopeptide antimicrobial agent with a potency significantly better than that of vancomycin when tested against staphylococci and streptococci. These two pathogens are common causes of skin and skin structure infections (SSSIs), and dalbavancin has been approved for the treatment of moderate to severe SSSIs. This study generated susceptibility data for staphylococci and beta-hemolytic streptococci from 52 U.S. medical centers that locally tested dalbavancin, vancomycin, and other antimicrobial class representatives to assess the potency of dalbavancin and the overall contemporary activities of commonly prescribed agents. Locally generated data showed that oxacillin-resistant staphylococci (57.0% overall) had slightly higher dalbavancin MIC(90) values (0.19 microg/ml) than oxacillin-susceptible strains (0.125 microg/ml). This potency was 8- to 16-fold greater than that for vancomycin. beta-Hemolytic streptococci had MIC(90) values ranging between <or=0.016 and 0.064 microg/ml (highest for group B isolates). Levofloxacin, gentamicin, and tetracycline were active against oxacillin-susceptible staphylococci (82 to 99% susceptible), with lower susceptibility rates seen for the oxacillin-resistant strains. Linezolid coverage was >98% against staphylococci. Erythromycin resistance was high for staphylococci (30.6 to 94.1%) with inducible clindamycin resistance rates of 26.0% and 55.0% for oxacillin-susceptible and -resistant Staphylococcus aureus, respectively. Beta-hemolytic streptococci had a 20.2% erythromycin resistance rate and a 60% inducible clindamycin resistance rate but were highly susceptible to other tested agents. Etest reading errors were apparent and skewed results towards slightly higher dalbavancin MICs, requiring further education on how to interpret Etest method results for this compound. Current disk diffusion breakpoint criteria for oxacillin susceptibility for S. aureus showed a very-major-error rate of 2.3% and only a 0.9% minor-error rate when cefoxitin was used to predict oxacillin susceptibility. Dalbavancin demonstrated excellent potency and activity (100% susceptibility at proposed breakpoints) against common causes of SSSI pathogens in this U.S. multicenter study sample.