Minimal Antibiotic Concentrations of Aminoglycosides and β-Lactam Antibiotics for Some Gram-Negative Bacilli and Gram-Positive Cocci

• Published in: The Journal of infectious diseases 1979-05, Vol.139 (5), p.599-603
• Main Authors: Lorian, Victor, De Freitas, Cicero Carlos
• Format: Article
• Language: English
• Subjects: Aminoglycosides; Aminoglycosides - pharmacology; Anti-Bacterial Agents; Antibacterials; Antibiotics; Bacteria; beta-Lactams - pharmacology; Cephalosporins; Dose-Response Relationship, Drug; Enterococcus faecalis - drug effects; Escherichia coli; Escherichia coli - drug effects; Gram positive cocci; Klebsiella pneumoniae - drug effects; Microbial Sensitivity Tests; Microbiology; Proteus mirabilis; Proteus mirabilis - drug effects; Pseudomonas aeruginosa - drug effects; Ratios; Staphylococcus aureus - drug effects
• ISSN: 0022-1899; 1537-6613
• DOI: 10.1093/infdis/139.5.599

The minimal antibiotic concentration (MAC) is the lowest concentration of an antibacterial agent that produces a decrease of 1 log in the number of organisms/ml as compared with a control culture in drug-free medium. Various gram-negative bacilli and gram-positive cocci were grown in the presence of amikacin, gentamicin, tobramycin, ampicillin, amoxicillin, oxacillin, carbenicillin, ticarcillin, and cefaman- dole at concentrations varying from eight times the minimal inhibitory concentration (MIC) to 1/128 of the MIC. Colony forming units (cfu) were counted, the MIC was determined, and the MIC: MAC ratio, which indicates the magnitude of the effective range, was calculated. The MIC: MAC ratio appears to be characteristic for a given species and antibiotic. There is no relation between the MICs and the MIC: MAC ratios. The highest ratios were given by Proteus mirabilis with aminoglycosides (MIC: MAC mean, 29.2 with tobramycin), and the lowest ratios were given with β-lactam antibiotics by Pseudomonas aeruginosa and Streptococcus faecalis (MIC:MAC means, 2.1 with carbenicillin and cefamandole, respectively).