Antagonistic effect of penicillin-amikacin combinations against enterococci

Pharmacokinetics of regional limb perfusion using a combination of amikacin and penicillin in standing horses

The objectives of this study were to evaluate the compatibility and the pharmacokinetic properties of combined amikacin and penicillin administration by intravenous regional limb perfusion (IVRLP) in horses. A tourniquet was applied proximal to the carpus of 7 clinically healthy adult horses and 2 g of amikacin and 10 × 10⁶ IU of penicillin (100 mL total volume) were sequentially injected into the cephalic vein just distal to the tourniquet. Synovial samples were collected from the joint at several times after injection. All samples were analyzed for amikacin and penicillin concentration. The mean maximum concentration (C_max) of both amikacin and penicillin was over 10-fold the relevant minimal inhibitory concentration (MIC) for all horses and remained above those MICs for at least 24 hours. The results of this study indicate that combining amikacin with penicillin during IVRLP in normal horses delivers high therapeutic synovial concentrations of both drugs.

EUCAST expert rules in antimicrobial susceptibility testing

EUCAST expert rules have been developed to assist clinical microbiologists and describe actions to be taken in response to specific antimicrobial susceptibility test results. They include recommendations on reporting, such as inferring susceptibility to other agents from results with one, suppression of results that may be inappropriate, and editing of results from susceptible to intermediate or resistant or from intermediate to resistant on the basis of an inferred resistance mechanism. They are based on current clinical and/or microbiological evidence. EUCAST expert rules also include intrinsic resistance phenotypes and exceptional resistance phenotypes, which have not yet been reported or are very rare. The applicability of EUCAST expert rules depends on the MIC breakpoints used to define the rules. Setting appropriate clinical breakpoints, based on treating patients and not on the detection of resistance mechanisms, may lead to modification of some expert rules in the future.

Gene homogeneity for aminoglycoside-modifying enzymes in gram-positive cocci

Aminoglycoside-resistant strains of Staphylococcus and Enterococcus, approximately 500 of each, were screened by dot blot hybridization for the presence of genes encoding aminoglycoside-modifying enzymes. The MICs of various aminoglycosides for the strains were determined, and the enzyme contents of the cells were inferred from the resistance phenotypes. The agreements (in percent) of the hybridization results with the deduced enzyme contents for Staphylococcus and Enterococcus species were, respectively, 80 and 87.6 for ANT(6) (aminoglycoside nucleotidyltransferase), 99.8 and 100 for both APH(3') (aminoglycoside phosphotransferase) and APH(2")-AAC(6') (aminoglycoside acetyltransferase), and 100 and 100 for ANT(4'). The weak correlation obtained with the probe for ANT(6) was due to the fact that gram-positive cocci can also be streptomycin resistant by synthesis of APH(3") or ANT(3")(9) and by ribosomal mutation. The remaining probes appeared to be specific: they hybridized with all the resistant clinical isolates but not with the susceptible strains. These results indicate that, except for streptomycin, nucleic acid hybridization is a valid approach for the detection and characterization of aminoglycoside resistance in gram-positive cocci.

Emergence of 4',4"-aminoglycoside nucleotidyltransferase in enterococci

Enterococcus faecium BM4102 was resistant to macrolide-lincosamide-streptogramin B-type (MLS) antibiotics; tetracycline-minocycline; and high levels of kanamycin, neomycin, tobramycin, and dibekacin but not gentamicin. This aminoglycoside resistance phenotype is new in enterococci. The genes conferring resistance to aminoglycosides and MLS antibiotics in this strain were carried on a plasmid, pIP810, that was self-transferable to to other Enterococcus strains. Resistance to tobramycin and structurally related aminoglycosides, kanamycin, neomycin, and dibekacin, was due to synthesis of a 4',4"-aminoglycoside nucleotidyltransferase. Homology was detected by hybridization between pIP810 DNA and a probe specific for a gene encoding an enzyme with identical site specificity in staphylococci. The bacteriostatic activity of amikacin apparently was not affected by the presence of the enzyme, although it was modified in vitro. However, the bactericidal activity of amikacin and the synergism of this aminoglycoside with penicillin were abolished.

Therapy of enterococcal infections

Because enterococci are typically tolerant of the bactericidal effects of cell wall-active antimicrobial agents, bactericidal therapy has required use of these agents in combination with aminoglycosides. For strains which do not demonstrate high-level aminoglycoside resistance, either streptomycin or gentamicin can be used in combination with penicillin, ampicillin or vancomycin. At some centers, as many as 50% of isolates display high-level gentamicin resistance. A minority of such isolates will not be highly streptomycin-resistant, and the latter drug can be used in combination with a cell wall-active drug. Optimal treatment of serious infections due to strains highly resistant to both streptomycin and gentamicin is unknown. While no agent is predictably bactericidal against such isolates, ampicillin, penicillin or vancomycin alone would be expected to cure some patients. Other drugs or drug combinations do not offer any predictable therapeutic advantages.

Antibiotic combinations: should they be tested?

When antibiotic combinations are used to provide a broader spectrum of antimicrobial activity or in an attempt to prevent the emergence of resistant organisms, it is rarely necessary or practical to perform tests of drug interactions in vitro. In vitro testing of combinations may be useful when combinations are used in an attempt to attain synergistic interactions. In some cases, screening methods can be used as substitutes for formal synergy testing. This paper examines the mechanisms of antibiotic interaction leading to synergism or antagonism, surveys attempts to correlate in vitro observations with efficacy in animal models, and reviews clinical data providing evidence for or against a useful role of synergistic antibiotic interactions in the treatment of human infections.

Penicillin-induced effects on streptomycin uptake and early bactericidal activity differ in viridans group and enterococcal streptococci

In vitro studies with penicillin and [3H]streptomycin in four strains of streptococci (S. faecalis, S. sanguis, and S. mitis) were performed by simultaneously measuring the rates of bacterial killing and uptake of streptomycin. In S. faecalis, penicillin stimulated streptomycin uptake, as has been shown by Moellering and Weinberg (R. C. Moellering, Jr., and A. N. Weinberg, J. Clin. Invest. 50:2580-2584, 1971). Moreover, the antibiotic combination was associated with an enhanced bactericidal rate which temporally correlated with beta-lactam-induced aminoglycoside uptake. In contrast, in viridans group streptococci (S. sanguis and S. mitis) penicillin had no effect on streptomycin uptake and a minimal effect on bactericidal rate when compared with either drug alone. These data suggested that the stimulation of streptomycin uptake in streptococci by penicillin is strain or species specific and that important differences exist between enterococci and viridans group streptococci regarding the mechanisms of beta-lactam-aminoglycoside potentiation.