Linking Phenotypic Categorization of Methicillin Resistant Staphylococcus aureus (MRSA) Cases with Genotypic and Clinical Characteristics

Background:

Methicillin resistant Staphylococcus aureus (MRSA) is a frequent cause of significant infections in the community and acute care settings.  As part of a surveillance project funded by the Center for Disease Control and  Prevention Epicenter Program, MRSA isolates from an academic medical center were collected between 1/2007 and 6/2010.  Antimicrobial susceptibility data were collated on each isolate and linked to clinical and genotypic characteristics.

Objective:

To determine if phenotypic patterns of antimicrobial susceptibility have predictable associations with genotypic and clinical characteristics.

Methods:

Susceptibility data on 7 antibiotic agents (vancomycin, clindamycin, tetracycline, rifampin, gentacmicin, linezolid, and trimethoprim-sulfa) were collated on MRSA isolates. Isolates were categorized as susceptible (resistant to none of 7 agents), intermediate (resistant to 1-2 of the agents) and resistant (resistant to ≥3 agents).  Molecular genotyping by repetitive element Polymerase Chain Reaction (rep-PCR;DiversiLab,^TM Biomerieux, NC) was conducted and pulse field gel electrophoresis (PFGE) types were derived from DiversiLab MRSA library and performed on selected representative repPCR isolates.  Rep-PCR strains were assigned arbitrary numeric classifications.  Clinical and demographic data were also collected on each isolate.

Results:

Susceptibility, genotyping, and clinical data were available on 723 MRSA isolates.  Phenotypic categories consisted of 363 (50%) as susceptible, 293 (41%) as intermediate, and 67 (9%) as resistant. The susceptible group had a mean age of 47.2;  were predominantly USA 300 PFGE type (256, 71%); and 258 (71%) were found in 5 rep-PCR groups (in descending order: types 9, 63, 13, 52, 2).   The intermediate group had a mean age of 57.6; were predominantly USA 100 PFGE type (258, 88%); and 182 (62%) were found in 4 rep-PCR groups (in descending order: types 2, 60, 65, 15). The resistant group had a mean age of 56.4; were predominantly Brazilian PFGE type (61, 91%); and 50 (75%) were found in 2 rep-PCR groups (in descending order: types 6, 53).  The predominant specimen type for the susceptible group was skin and soft tissue while blood was most common in intermediate and resistant groups

Conclusions:

MRSA strains with unique phenotypic profiles also appear to possess distinct genotypic and clinical patterns.  There was overlap in only one rep-PCR type among phenotypic groups.  Propagation of specific clones within populations is likely to result in predicted resistance patterns and clinical presentations.  Molecular and phenotypic classification of MRSA within populations can assist with selection of appropriate empiric antimicrobial therapy.  Routine molecular epidemiologic classification with rapid typing methods, such as rep-PCR, can reliably differentiate strains within phenotypic groups.