Antimicrobial susceptibilities and mechanisms of resistance of commensal and invasive Mycoplasma salivarium isolates

Abstract

<i>Mycoplasma salivarium</i>, an oral commensal organism, can cause severe invasive infections in immunocompromised individuals. Currently there is no treatment guidance for such infections. We performed antimicrobial susceptibility tests on 39 commensal and invasive <i>M. salivarium</i> isolates and investigated the mechanisms of antimicrobial resistance. Clindamycin was the most active agent [minimum inhibition concentration (MIC) range: 0.004-128 mg/L, MIC₅₀ = 0.031 mg/L, MIC₉₀ = 0.125 mg/ml], followed by tetracycline and levofloxacin. All isolates were resistant to erythromycin (MIC ≥4 mg/L) due to the presence of 2057A (<i>Escherichia coli</i> numbering) in 23S rRNA. Three isolates with elevated clindamycin MICs (≥8 mg/L) harbored A2058T/G mutations in <i>23S rRNA</i> gene; four sequential isolates from one patient developed C2611T and A2059G mutations accompanying the increase of clindamycin MICs. Five isolates with elevated tetracycline MICs (≥4 mg/L) had mutations in <i>16S rRNA</i> gene (A965G/T, G966T, or A967C/T) and one of them harbored <i>TetM</i>. Nine isolates with elevated levofloxacin MICs (≥4 mg/L) had one or more mutations in <i>gyrA</i>, <i>gyrB</i>, <i>parC</i>, or <i>parE</i>. Susceptibility breakpoints for clindamycin, tetracycline and levofloxacin were suggested to be ≤0.125, ≤2, and ≤2 mg/L, respectively. Antimicrobial resistance to any of the three agents (clindamycin, tetracycline, or levofloxacin) was documented in 12 (34.3%) non-duplicate isolates, of which 10 were invasive. Levofloxacin resistance was most frequent (25.7%). Multi-drug resistance was also observed (14.3%). This study demonstrates the frequent occurrence of antimicrobial resistance in <i>M. salivarium</i>, emphasizing the need for culture and susceptibility testing to guide antimicrobial therapy.