The effect of inoculum, culture medium and antimicrobial combinations on the in-vitro susceptibilities of Legionella pneumophila.

The in-vitro susceptibility of 14 strains of Legionella pneumophila to 17 anti-microbial agents were determined. The most active antibiotics were cefoxitin, erythromycin and doxycycline while the least active were cephalothin, cefamandole and cefazolin. Subculturing, type of medium and duration of incubation did not alter the effect of the antimicrobial agents. A change in the inoculum size, from 10(4) to 10(6) cfu, increased MICs two- to sixfold. Erythromycin was not synergistic with gentamicin, tobramycin or amikacin.