Discordance in Susceptibility to Mupirocin and Trimethoprim/Sulfamethoxazole in Colonizing Strains of Staphylococcus aureus Isolated from Different Body Sites

Background: Colonizing isolates of S. aureus serve as a reservoir for subsequent infection in self and others. Surveillance of antimicrobial resistance to agents used for decolonization or treatment is important for optimizing prevention and empiric management of S. aureus infections. Extra-nasal sites may be an important under-recognized reservoir for multi-drug resistant S. aureus.

Objective: We assessed the susceptibility patterns of methicillin resistant (MRSA) and methicillin susceptible (MSSA) strains colonizing nasal and extra-nasal sites of patients presenting for care at an urban academic Emergency Department (ED)

Methods: We previously reported a prevalence of 5% MRSA and 39% MSSA colonization in nasal and extranasal sites in 400 patients presenting to the Boston Medical Center ED. We performed additional studies on saved S. aureus isolates to determine the susceptibility to mupirocin (MUP), trimethoprim/sulfamethoxazole (TS), and vancomycin (VAN). Isolates were tested for the presence of mupA by PCR, and for MICs to MUP, TS, and VAN.  MIC breakpoints for MUP were sensitive (S) = ≤4 µg/ml, low level resistant (LLR) = 8 to 64 µg/ml, and high level resistant (HLR) = ≥ 512 µg/ml. Standard MIC breakpoints were used for TS and VAN.

Results: A total of 301 S. aureus isolates were recovered and saved from 400 patients. The distribution of sites included 104 nasal (from 99 subjects), 101 pharyngeal (from 96 subjects), 36 groin, 36 palms, 16 peri-rectal, and 8 skin/wounds.  None (0/301) of the isolates was I or R to VAN, positive for mupA, or HLR to MUP by MIC.  39 of 301 isolates were MRSA based on the identification of mecA by PCR.  None of the nasal MRSA (11) were MUP LLR, but 4 extra-nasal isolates were MUP LLR, including 1 groin and 3 peri-rectal isolates, for a prevalence of 4/39 (10%) MUP LLR among MRSA.  Of 263 MSSA, 9 (3%) isolates were MUP LLR; 1 nasal, 4 peri-rectal, and 4 skin/groin.  TS R was found in 5 nasal and 1 each of the pharyngeal, palm, groin, and peri-rectal MSSA isolates, for a MSSA TS R prevalence of 10/262 (4%). Among MRSA, 1 each of nasal, palm, and groin isolates and 2 peri-rectal isolates were TS R (13%). The nasal TS result was discordant from an extra-nasal TS result in 5 patients with MSSA and 1 patient with MRSA.

Conclusions: In over 300 colonization isolates, the presence of MUP LLR was predominant among peri-rectal MRSA and was discordant from the nasal MRSA MUP susceptibility. Thus nasal screening of MUP resistance may not be sufficient to assess the population prevalence of resistance to this agent. TS resistance was also discordant in nasal and extranasal sites. Since extranasal MRSA can contaminate the environment and be a source of infection, further assessment of these sites as a reservoir for multi-drug resistant MRSA is warranted.