Microtiter Broth Dilution Research Articles

347 β-LACTAMASE ACTIVITY & ANTIMICROBIAL SUSCEPTIBILITY OF HEMOPHILUS INFLUENZAE AND PARAINFLUENZAE IN MICH

The incidence of β-lactamase activity of 1000 Hemophilus strains collected Sept 76-Jan 78 from pediatric and adult patients throughout Michigan (utilizing an iodometric assay, AAC 7:265:1975) was 8/52 (15%) for type b, 29/318 (9%) for non-type b H. influenzae and 31/630 (5%) for H. parainfluenzae. From Sept 76-July 77 H. influenzae type b isolates were recovered from 16 children with meningitis; none were positive for β-lactamase activity. Subsequently 4/7 CSF isolates were positive for β-lactamase activity. The in vitro susceptibilities of 30 β-lactamase (+) Hemophilus strains to ampicillin, chloramphenicol, cefamandole, cefachlor, tetracycline and trimethoprim-sulfamethoxazole were evaluated using a microtiter broth dilution method. 70% of strains were resistant to [amp] of ≥10 μg/ml (MIC range = 4-128 μg/ml), while all were inhibited by ≥1 μg/ml of chloramphenicol (MIC=0.25-1 μg/ml). Strains were highly susceptible to cefamandole (90% inhibited by ≥0.5 μg/ml), although only 25% of strains were inhibited by ≥1 μg/ml of cefachlor (MIC-1-16 μg/ml). All isolates were inhibited by [tetracycline] of ≥1 μg/ml. MICs of TMP-SMZ ranged from .0037/.148-.125/2.37 μg/ml with 75% of strains susceptible to ≥0.03/.59 μg/ml. The incidence of β-lactamase (+) Hemophilus strains in Michigan appears to be increasing. These strains are highly susceptible in vitro to chloramphenicol, cefamandole and TMP-SMZ.

Quantitative antibiotic sensitivity determinations of Staphylococcus aureus isolated from eye cultures.

The sensitivities of 108 strains of Staphylococcus aureus, isolated from patients with a variety of ocular conditions, were tested quantitatively against nine antibiotics by means of a microtiter broth-dilution method. Of the nine antibiotics, cephalothin sodium had the lowest (and therefore the best) minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC). The MBC and MIC of gentamicin sulfate were very close to those of cephalothin. The MBCs and MICs of erythromycin, ampicillin sodium, penicillin, tetracycline, methicillin sodium, and carbenicillin disodium were moderately good. Chloramphenicol had the poorest response.

Antimicrobial susceptibility of Haemophilus parainfluenzae.

Fifty random clinical isolates of Haemophilus parainfluenzae were tested for their susceptibility to 10 antibiotics by a microtiter broth dilution method. Three of the strains tested were resistant to ampicillin, whereas eight were resistant to tetracycline. All strains tested were susceptible to chloramphenicol, kanamycin, gentamicin, cephalothin, and colistin. The ranges of minimal inhibitory concentrations for the three remaining antibiotics were: 0.5 to >/=128 mug of penicillin G per ml, 0.03 to 4 mug of carbenicillin per ml, and 1 to 16 mug of erythromycin per ml. Elevated minimal inhibitory concentrations for penicillin and carbenicillin were noted for the three ampicillin-resistant strains. Tests for beta-lactamase production demonstrated the presence of this enzyme in each of the three ampicillin-resistant strains.

Effects of Ampicillin-Amikacin and Ampicillin-Rifampin on Enterococci

Fifty-seven clinical isolates of enterococcus were tested for susceptibility to 10 antibiotics in a microtiter broth dilution system. Amoxicillin, ampicillin, vancomycin, and rifampin inhibited all strains at concentrations easily achievable in blood. Resistance to rifampin developed rapidly. Of the aminoglycosides, gentamicin was most active, followed in decreasing order by tobramycin, amikacin, kanamycin, and streptomycin. High-level resistance to streptomycin was present in 26% of the strains and to kanamycin in 23% of the strains. Growth curve studies of selected strains revealed synergy with ampicillin-amikacin and antagonism with ampicillin-rifampin. It is suggested that ampicillin-gentamicin constitutes adequate initial therapy for enterococcal infections until the results of tests for high-level resistance to kanamycin and streptomycin are known and that clinical trails of ampicillin-amikacin are warranted.

Antimicrobial Susceptibility of Haemophilus influenzae

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 mug of gentamicin per ml, 0.5 to 4 mug of erythromycin per ml, and 2 to 16 mug 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.

Antibiotic susceptibility of Streptococcus bovis and other group D streptococci causing endocarditis.

Seventy-four strains of Streptococcus bovis and 35 strains of enterococci (Streptococcus faecalis and its varieties, Streptococcus faecium and Streptococcus durans), most of which were isolated from patients with endocarditis, were tested for their susceptibility to penicillin, ampicillin, erythromycin, cephalothin, vancomycin, methicillin, tetracycline, chloramphenicol, kanamycin, streptomycin, and gentamicin. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) were determined by a microtiter broth dilution technique. All of these organisms are group D streptococci, but the S. bovis strains are not enterococci. On the basis of both MIC and MBC, the S. bovis strains were much more susceptibile in general to antibiotics then were the enterococcal strains. For the S. bovis strains, the lowest MICs were obtained with penicillin, ampicillin, and erythromycin, and the lowest MBCs with penicillin and ampicillin. Although these antibiotics were also the most active against the enterococci, the MICs and MBCs were much higher than obtained with the S. bovis strains. Gentamicin was the most active aminoglycoside. On the basis of in vitro susceptibility results, the S. bovis strains resemble the viridans streptococci rather than enterococci.

In vitro sensitivity of Salmonella to ten antimicrobial agents including sulfamethoxazole and trimethoprim, alone and in combination.

The activities of trimethoprim (TMP) and sulfamethoxazole (SMZ), alone and in combination (SMZ-TMP), and of the following antibiotics were tested against 115 clinical isolates of nontyphoid Salmonella species: tobramycin, gentamicin, ampicillin, amoxicillin, neomycin, kanamycin, chloramphenicol, and tetracycline. The methods of disk diffusion, microtiter broth dilution, and agar dilution were employed for all single antimicrobial agents as well as for SMZ-TMP studies. Growth curves were performed in broth. SMZ-TMP, TMP, gentamicin, tobramycin, and neomycin were the most active drugs in vitro. All strains were inhibited by </=1 mug of TMP per ml, but >100 mug of SMZ per ml was required for at least 10% of strains. SMZ and TMP in a ratio of 10:0.5, respectively, inhibited all isolates and were synergistic for 105 strains. All strains inhibited by the combination of 10:0.5 SMZ-TMP had a zone diameter of >/=22 mm by using a combination disk containing 1.25 mug of TMP and 23.75 mug of SMZ. Seven isolates were resistant to >100 mug/ml of ampicillin or amoxicillin; all isolates were sensitive to chloramphenicol at </=6.3 mug/ml. SMZ-TMP appears to be active against nontyphoid salmonellae in vitro; this is usually due to a synergistic effect.

Interpretation of diffusion susceptibility data obtained with 50-mu g carbenicillin discs against gram-negative organisms.

A total of 284 clinical isolates of various species of Enterobacteriaceae, Pseudomonas aeruginosa, P. maltophilia, and Acinetobacter anitratum were tested for susceptibility to carbenicillin by the standardized Bauer-Kirby disc diffusion technique and a microtiter broth dilution method. The data obtained led to the following proposed criteria for the interpretation of the results of disc susceptibility tests. Enterobacteriaceae that yield zones of inhibition equal to or greater than 20 mm in diameter around 50-mug discs of carbenicillin are designated as sensitive to the drug; isolates that yield zones measuring from 18 to 19 mm in diameter are reported as of equivocal (intermediate) susceptibility to the drug, whereas those enterobacterial isolates that are characterized by zones of inhibition of 17 mm or less in diameter are interpreted as resistant to carbenicillin. Isolates of P. aeruginosa, P. maltophilia, and A. anitratum yielding zones of 14 mm or more in diameter around 50-mug discs of carbenicillin are reported as sensitive, whereas those isolates that are characterized by zones of 13 mm or less in diameter are reported as resistant to this drug.

Interpretation of Diffusion Susceptibility Data Obtained with 50-μg Carbenicillin Discs Against Gram-Negative Organisms

A total of 284 clinical isolates of various species of Enterobacteriaceae, Pseudomonas aeruginosa, P. maltophilia, and Acinetobacter anitratum were tested for susceptibility to carbenicillin by the standardized Bauer-Kirby disc diffusion technique and a microtiter broth dilution method. The data obtained led to the following proposed criteria for the interpretation of the results of disc susceptibility tests. Enterobacteriaceae that yield zones of inhibition equal to or greater than 20 mm in diameter around 50-mug discs of carbenicillin are designated as sensitive to the drug; isolates that yield zones measuring from 18 to 19 mm in diameter are reported as of equivocal (intermediate) susceptibility to the drug, whereas those enterobacterial isolates that are characterized by zones of inhibition of 17 mm or less in diameter are interpreted as resistant to carbenicillin. Isolates of P. aeruginosa, P. maltophilia, and A. anitratum yielding zones of 14 mm or more in diameter around 50-mug discs of carbenicillin are reported as sensitive, whereas those isolates that are characterized by zones of 13 mm or less in diameter are reported as resistant to this drug.