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Quantitative Assessment of Bactericidal Activities of β-Lactam Antibiotics by Agar Plate Method
Author(s) -
Gohta Masuda,
Susumu Tomioka
Publication year - 1978
Publication title -
antimicrobial agents and chemotherapy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.07
H-Index - 259
eISSN - 1070-6283
pISSN - 0066-4804
DOI - 10.1128/aac.14.4.587
Subject(s) - microbiology and biotechnology , serratia marcescens , minimum inhibitory concentration , staphylococcus epidermidis , staphylococcus aureus , pseudomonas aeruginosa , minimum bactericidal concentration , agar plate , agar , antibiotics , biology , klebsiella pneumoniae , enterococcus , streptococcus , agar dilution , enterobacter , bacteria , streptococcus pneumoniae , escherichia coli , biochemistry , genetics , gene
Quantitative bactericidal activities of β-lactam antibiotics were determined by the agar plate method. Broth cultures, of which the colony-forming units were counted before the study, were inoculated on antibiotic-containing agar plates, utilizing a 10−3 , 10−2 , or 10−1 dilution or undiluted culture plated with each 0.001-ml calibrated loop. These plates were incubated at 37°C overnight, and the minimal drug concentration at which no bacterial growth was observed on the plates was defined as minimal inhibitory concentration. After this procedure, the agar surface was treated with β-lactamase spray to inactivate the antibiotic. These plates were incubated again at 37°C overnight. The minimal drug concentration at which no evidence of bacterial growth was visible on the plates (resulting in a 100% kill) was defined as minimal bactericidal concentration. The lowest concentration which reduced the number of colony-forming units to 1/1,000 that in the original inoculum (resulting in a 99.9% kill) was defined as minimal lethal concentration. When compared forEscherichia coli, Klebsiella pneumoniae, Enterobacter sp.,Serratia marcescens, Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermidis , alpha-hemolytic streptococcus (non-enterococcal), beta-hemolytic streptococcus, and enterococcus, the minimal bactericidal concentrations were generally several fold higher than the minimal inhibitory concentrations. Minimal lethal concentrations were virtually the same as minimal inhibitory concentrations for gram-negative strains; however, for some gram-positive strains, minimal lethal concentrations were higher than minimal inhibitory concentrations.

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