A rapid capillary tube method was used to demonstrate beta-lactamase production by ampicillin-resistant strains of Haemophilus influenzae .

The effect of calcium and magnesium on the susceptibility of 13 species of Pseudomonas to tetracycline, gentamicin, polymyxin B, and carbenicillin was measured. The majority of the minimum inhibitory concentrations (MICs) of these antibiotics was increased if these cations were added to the test media. The increases in MICs caused by calcium or magnesium were similar, but the combination of both -ions generally caused a greater change than either alone. Although the MIC of polymyxin B was most affected by calcium and magnesium, its interpretive susceptibilities (i.e., whether susceptible or resistant) were least changed. Susceptibility tests on Pseudomonas species probably should be done with Muller-Hinton broth supplemented with physiological concentrations of calcium and magnesium to better approximate the in vivo activity of these antibiotics. When the susceptibility tests were performed with Mueller-Hinton agar, the MICs were slightly less than those obtained with Mueller-Hinton broth supplemented with both cations but greater than those obtained with MuellerHinton broth supplemented with individual cations.

Selected clinical isolates of Haemophilus influenzae were tested for their susceptibility to seven antibiotics by a microtiter broth dilution and an agar diffusion method. Eleven of 40 strains tested were resistant to ampicillin, the drug of choice, by both methods. All the strains tested were susceptible to chloramphenicol, and all but one were susceptible to tetracycline. Of the other four antibiotics tested, the ranges of minimal inhibitory concentrations (MICs) were: 0.5 to 4 pg of gentamicin per ml, 0.5 to 4 ,ug of erythromycin per ml, and 2 to 16 pg of clindamycin per ml. MICs of ampicillin for both the susceptible and resistant strains were markedly affected by inoculum concentration. The ampicillin MICs of the resistant strains were also affected by the time of incubation.

Six isolates of Legionnaires disease bacteria were tested for their susceptibility to 22 antimicrobial agents. The most active agent was rifampin (minimal inhibitory concentration, ≤0.01 μg/ml). On the basis of minimal inhibitory concentration breakpoints that have been used to categorize susceptibility for most of these drugs, the organisms were susceptible to rifampin, cefoxitin, erythromycin, the aminoglycosides, minocycline and doxycycline, chloramphenicol, ampicillin, penicillin G, carbenicillin, colistin, and sulfamethoxazole-trimethoprim (19:1 ratio); sensitive to intermediate in susceptibility to tetracycline, methicillin, cefamandole, cephalothin, and clindamycin; and resistant to vancomycin. More clinical data must be obtained before an optimal therapeutic regimen can be recommended.

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