Comparison of identification and antimicrobial resistance pattern of Staphylococcus aureus isolated from Amassoma,...

Staphylococcus aureus is often responsible for fatal infections and recent upsurge of resistant strains has resulted in therapeutic failure. The identification of this microorganism is a major challenge to medical microbiologists in developing countries.

One hundred and eighty five isolates which had been previously isolated from the nares of 185 healthy college students' volunteers in Amassoma, Bayelsa State, South Nigeria were identified by MALDI TOF mass spectrometry, and PCR amplification of the spa gene. The identified isolates were compared with presumptive identities obtained by growth on MSA, tube coagulation and slide agglutination tests. Antimicrobial susceptibility testing of S. aureus isolates was performed by Kirby Bauer technique while MRSA was screened for by growth on chromlDTM MRSA plate and confirmed by PCR-amplification of mecA/mecC genes.

From the 185 staphylococci that grew with yellow colonies on MSA, 24 were positive in the slide coagulase test, while 17 were positive in the tube coagulase test; MALDI TOF mass spectrometry and PCR amplification of the spa gene showed excellent concordance with the tube test, as all tube coagulase-positive strains were identified as S. aureus, while tube coagulase-test negative isolates in all cases were designated as other staphylococcal species by MALDI-TOF mass spectrometry and were spa PCR test negative. All S. aureus isolates were susceptible to clindamycin, vancomycin, fusidic acid, rifampicin and linezolid, while observed resistance to penicillin and trimethoprim were high. Only one MRSA strain was detected.

The study confirms that the tube coagulase test is an accurate diagnostic method for identification of S. aureus, while growths on MSA and slide agglutination tests are inaccurate. We found a low prevalence of MRSA and a high rate of trimethroprim-resistance in the studied population.