Expression of resistance to tetracyclines in strains of methicillin-resistant Staphylococcus aureus

A diverse collection of methicillin-resistant Staphylococcus aureus (MRSA) isolates resistant to tetracycline was screened by PCR for the presence of the resistance determinants tetK, tetL, tetM or tetO. Twenty-four of 66 iso lates had tetM alone, 21 had tetK alone and 21 had both tetK and tetM (tetK M). All isolates were tetL- and tetO-negative. MICs of tetracycline, doxycy cline and minocycline were evaluated for all isolates with or without prein cubation in the presence of subinhibitory concentrations of tetracycline or minocycline. All isolates with one or more tetracycline resistance determi nants were resistant to tetracycline 8 mg/L without induction of resistance . Some MRSA isolates of each of these three genotypes showed an unexpected lack of resistance to tetracyclines when the disc diffusion or agar dilutio n method was applied to uninduced cells. Resistance to tetracycline and dox ycycline was greater (two- to four-fold) in tetK cells preincubated with te tracycline (tetK MRSA isolates were susceptible to minocycline less than or equal to 0.25 mg/L under all conditions tested). For isolates with tetM al one, preincubation with tetracycline or minocycline gave up to a four-fold increase in the level of resistance to doxycycline and minocycline. Inducti on of doxycycline and minocycline resistance was clearly observed for tetKM Isolates when cells were preincubated with minocycline. This study suggest s that, despite the results of susceptibility testing, all tetracycline-res istant S. aureus isolates should be treated as resistant to doxycycline, an d all tetM-positive isolates should be treated as resistant to all tetracyc lines. A double disc diffusion method has been developed to identify induci ble resistance to minocycline and to distinguish between tetK, tetM and tet KM isolates.