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Antibiotic Susceptibility of Staphylococcus Aureus with VanA and MecA Genes

Ghimire L, Banjara MR, Abdulla AM

J Nepal Health Res Counc, 2024 Mar 31;21(4):616-622.
PMID: 38616592 DOI: 10.33314/jnhrc.v21i4.4861

BACKGROUND: Staphylococcus aureus (S.aureus) is an emerging antibiotic resistant bacterium responsible for various infections in human. Resistance to methicillin and vancomycin are of prime concern in S. aureus. The study aims to determine the minimum inhibitory concentration (MIC) of Vancomycin and evaluate the existence of mecA and vanA genes, associated with antibiotic resistance.
METHODS: Clinical specimens from three Kathmandu hospitals were processed and S. aureus was identified using conventional microbiological procedures. MRSA was phenotypically identified with cefoxitin (30µg) disc diffusion, while vancomycin susceptibility was assessed using the Ezy MICTM stripes. The mecA and vanA genes were detected by polymerase chain reaction (PCR).
RESULTS: Out of 266 S. aureus samples from various clinical specimen subjected for analysis, 77 (28.9%) were found methicillin-resistant (MRSA) and 10 (3.8%) were observed vancomycin-resistant (VRSA). Vancomycin resistant isolates showed a significant correlation between resistance to ampicillin, chloramphenicol, and cefoxitin. The mecA gene was found in 39 of the MRSA isolates, having 50.64% of MRSA cases, while the vanA gene was detected in 4 of the VRSA cases, constituting 40% of VRSA occurrences.
CONCLUSIONS: The strains with higher vancomycin minimum inhibitory concentration values (≥ 1.5 μg/ml) displayed increased resistance rates to various antibiotics compared to strains with lower minimum inhibitory concentration values (< 1.5 μg/ml). The presence of vanA genes was strongly associated (100%) with vancomycin resistance, while the 10.3% mecA gene was identified from MRSA having resistance towards vancomycin also.

Matched MeSH terms: Cefoxitin/pharmacology
Underdiagnosis of Borderline oxacillin-resistant Staphylococcus aureus (BORSA) - Case series

Samsudin N, Chua WC, Hasan H, Hassan SA, Deris ZZ

Malays J Pathol, 2024 Apr;46(1):95-102.
PMID: 38682849

Borderline oxacillin-resistant Staphylococcus aureus (BORSA) are mecA-negative strains with oxacillin minimum inhibitor concentration (MIC) close to the resistance breakpoint of ≥ 4μg/mL. Instead of producing penicillin-binding protein with low affinity to methicillin (oxacillin) mediated by mecA gene as in methicillin-resistant S. aureus (MRSA), BORSA strains are characterised by the hyperproduction of β-lactamase enzymes, thus able to break down methicillin. Common laboratory methods to detect MRSA such as cefoxitin disk diffusion alone may fail to detect methicillin resistance due to BORSA. We report five cases of BORSA blood-stream infections in a university teaching hospital. All isolates were found to be susceptible to cefoxitin using disk diffusion, resistant to oxacillin using automated MIC method, and did not harbour mecA gene. All patients were suscessfully treated with anti-MRSA antibiotics, and removal of primary sources were done if identified. A more cost-effective method for screening and diagnosis of BORSA is needed in addition to cefoxitin disk diffusion test, in order to monitor the spread, and to enable routine detection and treatment of this pathogen.

Matched MeSH terms: Cefoxitin/pharmacology
Comparative evaluation of five culture media with triplex PCR assay for detection of methicillin-resistant Staphylococcus aureus

Al-Talib H, Yean CY, Al-khateeb A, Singh KK, Hasan H, Al-Jashamy K, et al.

Curr Microbiol, 2010 Jul;61(1):1-6.
PMID: 20033170 DOI: 10.1007/s00284-009-9567-8

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) is responsible for nosocomial and community-acquired infections. Hence, rapid and accurate laboratory diagnosis of MRSA is a vital constituent of control measures. The present study evaluated five different methods for the identification of MRSA. A total of 207 S. aureus clinical isolates that consisted of 89 MRSA and 118 methicillin-susceptible S. aureus (MSSA) strains confirmed by PCR were tested. MRSA strains were evaluated by five different methods: chromogenic MRSA agar (CMRSA), oxacillin resistance screening agar base (ORSAB), mannitol salt oxacillin agar (MSO), mannitol salt cefoxitin agar with two different concentrations of cefoxitin [4 microg/ml (MSC-4) and 6 microg/ml (MSC-6)]. The results of the different methods were compared to mecA PCR as the gold standard. MSC-6 showed only six false-positive MRSA in comparison with PCR. The sensitivities and specificities of MSC-6, MSC-4, MSO-4, ORSAB, and CMRSA were as follows: 98.9/94.9%, 100/83.1%, 89.9/87.3%, 97.8/96.6%, and 95.5/94.9%, respectively. In comparison with PCR, it was found that both MSC-6 and ORSAB were relatively the least expensive screening tests ($0.70 and $1.00, respectively). In conclusion, all methods were comparable, but MSC-6 was the least expensive medium for MRSA screening.

Matched MeSH terms: Cefoxitin/pharmacology
Extended-spectrum beta-lactam resistance due to AmpC hyperproduction and CMY-2 coupled with the loss of OMPK35 in Malaysian strains of Escherichia coli and Klebsiella pneumoniae

Palasubramaniam S, Subramaniam G, Muniandy S, Parasakthi N

Microb Drug Resist, 2007;13(3):186-90.
PMID: 17949305

In this report, we describe the detection of AmpC and CMY-2 beta-lactamases with the loss of OmpK35 porin among seven sporadic strains of ceftazidime-resistant Klebsiella pneumoniae and ceftazidime-resistant Escherichia coli. Cefoxitin, which was used as a marker of resistance toward 7-alpha-methoxy-cephalosporins, exhibited high minimum inhibitory concentration (MIC) values ranging between 128 microg/ml and >256 microg/ml in all the strains. The presence of hyperproducing AmpC enzymes was indicated by the positive three-dimensional test. Isoelectric focusing (IEF) study confirmed the presence of AmpC enzymes in all the strains. The ampC gene was detected by PCR in all the strains and confirmed by DNA sequencing. Large plasmids in all the strains, ranging from 60 kb to 150 kb in size, most likely encode the ampC gene. Two E. coli strains out of the seven strains showed positive amplification of the bla(CMY-2) gene, an AmpC variant, and was confirmed by DNA sequence analyses. DNA hybridization confirmed the bla(CMY-2) gene to be plasmid-mediated in both of these strains. However, one of these two strains also mediated a chromosomal CMY gene. All the strains showed an absence of OmpK35 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS/PAGE) and was confirmed by western blot analyses using raised polyclonal anti-OmpK35 antiserum. This suggests that, apart from CMY production, absence of OmpK35 porin also contributed to cefoxitin resistance resulting in extended-spectrum beta-lactam resistance among these isolates.

Matched MeSH terms: Cefoxitin/pharmacology