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KMID : 0376219840210030503
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Chonnam Medical Journal
1984 Volume.21 No. 3 p.503 ~ p.511
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Effects of Ampicillin and Methicillin combination on the Growth of ~-lactamase-positive Staphylococcus aureus
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徐明秀/Suh, Myung Soo
柳必悅/吳宗錫/安泰烋/Ryu, Phil Yul/Oh, Jong Suk/Ahn, Tai Hew
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Abstract
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Combined antibiotics have recently been applied in medicine to control drugresistant organisms. PC-resistant Staphylococcus aureus and Pseudomonas aeruginosa as opportunist have most frequently been submitted as the target cells because of their notoriousness as nosocomial agents.
Numerous experimental data are available as to the combined action of various antibiotics, the effects being variable from synergism to antagonism. The mechanism has, however, been controversial; evaluating simply through single drug´s inhibitory mechanism has not always been leading to the expected results.
As an attempt to solve one of the aspects of complicated mechanisms concerned with the combined drug action, b-lactamase- positive strains of Staphylococcus aureus were submitted, in the present experiment, to the combined action of AP and MT while b-lactamase -negative (PC-resistant) ones being tested as control.
Fifteen strains of b-lactamase -positive Staphylococcus aureus and 5 b-lactamase negative ones were chosen randomly from 68 strains most recently isolated.
Susceptibility test showed that most of the b-lactamase-positive strains were highly resistant to PC(MIC>125U/ml) while the b-lactamase-negative ones being moderately resistant (MIC=63-32); MX, AP and MT did not reveal any particular differences in their MIC distributions, mostly being from moderate to low degrees of resistance.
Combination of AP and MT, however, significantly inhibited the growth of b-latamase-positive strains, i. e. , synergism was observed in 12 strains out of 15, while only additive or indifferent effect was observed in b- lactamase -negative strains(FICI: 0.75-1.0). Thus, the presence of b-lactamase seems to play a great role in the synergic effect of AP and MT combination. The resistance of MT to b-lactamase could not, however, be the main cause of growth inhibition, since the original concentration of each drug in the combination was the MIC of the drug when used singly.
Thus the differences observed between b-lactamase-positive and negative strains in combined antibiotic action, however, suggest that plasmid-derived enzymatic factors concerned with the drug resistance may become more easily ineffective than factors derived from chromosomal mutation by the combined action of antibiotics.
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