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The effects of imipenem on the growth of Escherichia coli ATCC 25922 were studied using a bioluminescence assay of bacterial ATP, microscopy and viable counting in iso-osmotic Mueller-Hinton broth (MHB) and hypo-osmotic nutrient broth (NB). Imipenem showed a post-antibiotic effect (PAE) of > 2 h for E. coli in both MHB and NB after 2 h exposure to 1 and 8 mg/L of imipenem when determined by bioluminescence and microscopy. The intracellular ATP level increased after 2 h exposure of E. coli to 1 mg/L of imipenem in MHB. In this culture there was a predominance of spheroplasts. These spheroplasts were large and osmotically fragile and a 10 min treatment in water-diluted MHB (hypo-osmotic) prior to the assays lysed the large spheroplasts. This reduced the intracellular ATP level and shortened the PAE when determined by bioluminescence, and caused more rapid initial killing and a negative PAE when determined by viable counting. At 8 mg/L imipenem in MHB and at all concentrations in NB there was a predominance of rods and only a small number of spheroplasts which all disappeared when the cultures resumed logarithmic growth. In these cultures there was a significant initial decrease in intracellular ATP. This study showed reasonable agreement between microscopy and bioluminescence, which are direct methods, for determining the initial killing and PAE of imipenem on E. coli. More rapid initial killing and shorter or no PAEs, were in general, obtained in both MHB and NB when determined by viable counting. However, the effective regrowth time, defined as the time for the bacterial density to increase 1 log10 from the pre-exposure inoculum, was independent of the method used for measuring regrowth in both MHB and NB.

Three hundred and sixteen patients with serious infections verified or suspected to be of Gram-negative aetiology were treated in an open, randomized, comparative multicentre study with amikacin 15 mg/kg/day given either as a single dose or in two divided doses at 12 h intervals. Two hundred patients were evaluated for efficacy and all 316 for safety. The efficacy of both dosage regimens was very good with a satisfactory clinical response in 90% of the patients. There were no significant differences between the two regimens regarding efficacy and safety. This was also confirmed in an analysis according to the principle of 'intention-to-treat' including all randomized patients. In 218 patients additional therapy, most commonly with piperacillin or ampicillin, was considered necessary. The mean peak serum concentration of amikacin was 40.9 mg/L in the once-daily group, which is 10 x MIC for most Gram-negative bacteria, compared to 24.4 mg/L in the twice-daily group, which is 6 x MIC. Mean trough serum concentrations after 24 h were 1.8 mg/L in the once-daily group and 3.1 mg/L after 12 h in the twice-daily group. These serum concentrations were often close to or just below the MICs of the isolated pathogens. Drug related adverse reactions were seen in 40 (13%) of the patients. Among the adverse reactions with possible or probable relation to amikacin were 20 nephrotoxic events, nine in the once-daily group and 11 in the twice-daily group. A multivariate analysis of selective causative factors and nephrotoxic events gave a low correlation for once- vs twice-daily amikacin therapy. Five ototoxic events were observed, three in the once-daily group and two in the twice-daily group. One patient in the once-daily group experienced nausea in connection with amikacin infusions.

Selection and regrowth of resistant variants, which are present in low frequencies in the initial inoculum, were seen when large inocula of five strains of Staphylococcus aureus and four strains of Staphylococcus epidermidis were incubated in broth with amikacin, gentamicin, netilmicin and tobramycin. Statistical analysis showed no significant difference between the aminoglycosides in the selective growth of resistant variants (P0.5). Vancomycin differed significantly from the aminoglycosides in both the frequency of, and selection of resistant variants (P0.001). No bacteria resistant to1 x MIC was seen in the vancomycin-exposed cultures of S. aureus and S. epidermidis, while in most aminoglycoside-exposed cultures, bacteria resistant to 4-16 x MIC were seen.

The in-vitro post-antibiotic effect (PAE) of amikacin alone and in combination with ceftazidime, ceftriaxone and piperacillin was studied for two strains of Enterococcus faecalis using a bioluminescent assay of bacterial ATP. The two strains of E. faecalis were resistant to amikacin, ceftazidime and ceftriaxone but sensitive to piperacillin. The bacterial cultures were incubated with the beta-lactam antibiotics for 1 h and concentrations of amikacin between 2-64 mg/l were then added. Thereafter, incubation continued with the combinations for one more hour. After dilution, regrowth was monitored by measuring bacterial ATP every hour. Increasing concentrations of amikacin (2-64 mg/l), ceftazidime (8-32 mg/l) and ceftriaxone (32-128 mg/l) resulted in little or no PAE (0-0.3 h) on these strains. PAEs of 0.5 to 1.6 h resulted from exposure to piperacillin (4-32 mg/l). In combination amikacin and piperacillin increased the PAE to 5.5 h. A synergistic PAE was also seen when the enterococci were exposed to amikacin combined with ceftazidime or ceftriaxone in concentrations close to the MICs of the latter antibiotics.

The synergic in-vitro post-antibiotic effect (PAE) of mecillinam, in combination with either ampicillin, aztreonam, ceftazidime or piperacillin, on a reference strain of Escherichia coli was evaluated by bioluminescence assay of bacterial ATP. Ampicillin, ceftazidime and mecillinam alone induced a concentration dependent PAE (greater than 3 h) on E. coli, whereas aztreonam and piperacillin alone induced a short (less than 1 h) non-dose dependent PAE. At most concentrations, the combination of mecillinam and ampicillin, aztreonam, ceftazidime or piperacillin induced longer PAEs on E. coli than the sum of the individual antibiotics' PAEs. Long PAEs were seen concomitantly with the presence of spheroplasts. In addition to the synergistic PAE, the decrease in colony counts and changes in ATP values after a 2 h exposure to mecillinam, in combination with the other beta-lactam antibiotics, were more prominent than the respective values after exposure to the individual antibiotics. The change in ATP was generally less pronounced than the decrease in colony counts. This could be due to lysis of spheroplasts on agar plates, leading to an over-estimation of the initial killing when assayed by viable counting. Mecillinam, which induced long PAEs on E. coli at almost all concentrations in this study, has a high affinity for penicillin binding protein 2 (PBP 2) and induced spheroplast formation at all concentrations. However, mechanisms other than the affinity for PBP 2 and spheroplast formation are involved in the PAE of beta-lactam antibiotics on Gram-negative bacteria; since the PAE was prolonged when mecillinam was combined with ampicillin, aztreonam, ceftazidime or piperacillin, which bind preferentially to PBP 1 and 3.

Two hundred and twenty patients with serious infections verified or suspected to be of Gram-negative aetiology were treated in an open randomized comparative multicentre trial with amikacin 15 mg/kg/day given either as a single dose or in two divided doses at 12-h intervals. Amikacin was administered as a short-term iv infusion. When additional therapy was considered necessary piperacillin or ampicillin was recommended. The trial continues and an interim report on data from the 12 participating Scandinavian hospitals is presented. One hundred and forty-four patients have been evaluated for efficacy and 213 patients for safety. There were no significant differences between the two dosage regimens regarding efficacy and safety. A satisfactory clinical response was recorded in 129 (90%) of the evaluable patients. One serious adverse reaction was seen in a patient in the once-daily group. This was ototoxicity which was superimposed on a long standing hearing defect possibly caused by previous streptomycin therapy.

Forty-five mostly elderly patients with serious infections were treated in a prospective, comparative and randomized pharmacokinetic study with amikacin 11.0 or 15.0 mg/kg administered in a single daily dose as an intravenous, short-term infusion or with amikacin 7.5 mg/kg administered twice daily in the same way. The results indicate that administration of amikacin 15 mg/kg in a single daily dose should be a practical and safe principle of administration. However elderly patients often have reduced creatinine clearance and should preferably be given a lower dose of 11 mg/kg bw. The risk of nephrotoxicity did not increase, but conclusions on ototoxicity and clinical efficacy cannot be drawn from this limited study. This should be considered as an initial part of a future multicentre trial.