Serum inhibitory and bactericidal activity against methicillin-resistant Staphylococcus aureus in volunteers receiving novobiocin and rifampin alone and in combination.

The combination of novobiocin and rifampin is effective in eliminating colonization due to methicillin-resistant Staphylococcus aureus (MRSA) and in treating experimental MRSA soft tissue infections. To evaluate novobiocin, rifampin, and the combination of the two agents for potential oral therapy in patients with MRSA infections, we measured the serum inhibitory and bactericidal activity from four volunteers against 20 MRSA strains obtained from seven different institutions. When Stratton-Reller methods employing 50% human serum were used to perform the assay, rifampin produced peak mean serum inhibitory titers of 1:40, whereas novobiocin alone produced essentially no inhibitory activity. The combination of novobiocin plus rifampin had similar inhibitory activity as rifampin alone. The bactericidal titers produced by the three regimens were significantly less than inhibitory titers. In additional studies, involving serum from five volunteers tested against seven representative strains, peak mean serum inhibitory activity of novobiocin was 1:232 when Mueller-Hinton broth was used as the diluent compared with < 1:2 when 50% human serum was used. We conclude that despite the high degree of activity of novobiocin in broth, its activity against MRSA in serum is minimal, probably related to the high degree of protein binding of that antibiotic.

Technical aspects and clinical correlations of the serum bactericidal test.

A review of the studies using 50% human serum as a diluent for the serum bactericidal test has shown correlations with patient outcome. Human serum used as diluent of the patient's serum appears to be essential because of high protein binding of some antibiotics. An inoculum of 10(5)-10(6) bacteria/ml and a bactericidal criteria of 99.9% killing are technical aspects that have gained popularity. Careful timing of serum collection for the assay is important. Neither the macrotube nor microtiter techniques are entirely satisfactory. The latter method, however, has the advantage of being more reproducible than the macrotube method, less cumbersome and requiring less serum. Preliminary guidelines for performing and interpreting the test are provided. Future research should be directed toward making the microtiter technique more sensitive for identifying antibiotic tolerance, developing effective methods to eliminate the need for human serum as a diluent and obtaining more clinical correlations.

Novobiocin and rifampicin in combination against methicillin-resistant Staphylococcus aureus: an in-vitro comparison with vancomycin plus rifampicin.

Novobiocin and rifampicin were evaluated in vitro as a possible new antibiotic combination against methicillin-resistant Staphylococcus aureus. An evaluation of 20 strains of methicillin-resistant Staph. aureus using microdilution checkerboard techniques at 10(5) cfu/ml showed neither synergy nor antagonism between novobiocin and rifampicin or between vancomycin and rifampicin. Agar surface inoculation of six strains of methicillin-resistant Staph. aureus showed increased synergy with increased inocula (10(6)-10(9) cfu) for novobiocin plus rifampicin compared to vancomycin plus rifampicin. Time-kill curves showed indifference at 6h for all combinations, whereas, at 24 and 48 h, they generally showed indifference, occasionally synergy, but never antagonism. The 'synergy' between novobiocin and rifampicin at higher inocula of methicillin-resistant Staph. aureus appears to be due to prevention of emergence of resistant organisms and may have clinical relevance. The combination of novobiocin-rifampicin merits further investigation.

Measurement of amikacin in serum by a latex agglutination inhibition test.

Levels of amikacin in serum were determined in 106 serum specimens by a latex agglutination inhibition card test and by radioimmunoassay (RIA). Linear regression analysis demonstrated a high degree of correlation between the two assays (latex = 0.95 (RIA) + 0.69; r = 0.97). Assay of three control sera containing 7.5, 15, and 30 micrograms of amikacin per ml on 7 separate days showed good reproducibility with a coefficient of variation of 0 to 11.7% for the latex assay compared with 7.01 to 22.2% for RIA. Recovery of amikacin in spiked sera varied between 93 and 108% for the latex assay compared with 90 and 100% for RIA. Because the procedure involves a titer, the latex agglutination inhibition card test produces results which are categorized rather than results which are continuous. However, it is a rapid and specific method for determining amikacin levels in clinical specimens and is particularly useful when processing small numbers of specimens.

Comparative pharmacokinetics and serum bactericidal activity of mezlocillin, ticarcillin and piperacillin, with and without gentamicin.

We compared the serum levels, pharmacokinetics and serum bactericidal activity after intravenous infusions of 5 g of each of three anti-pseudomonal penicillins--ticarcillin, mezlocillin and piperacillin alone and in combination with 80 mg gentamicin in normal volunteers. Gentamicin levels were significantly lower when administered with ticarcillin than when administered with mezlocillin or piperacillin. Serum levels of ticarcillin and piperacillin immediately after the infusion were similar (approximately 450 mg/l) and were higher than mezlocillin at 350 mg/l, but by 6 h, levels of all three penicillins were less than 10 mg/l. Overall, the geometric mean bactericidal titres produced in the serum against organisms which commonly infect cancer patients with granulocytopenia were highest with the piperacillin-gentamicin combination. However, except for the mezlocillin-gentamicin combination against Pseudomonas aeruginosa, serum bactericidal titres of all three penicillin-gentamicin regimes were greater than 1:8 at zero and 2 h after the infusion against the organisms tested. It is unlikely that these differences in the three penicillin-gentamicin combinations will be apparent in the outcome of clinical trials for empiric therapy of fever in the cancer patient with granulocytopenia.

Activity of novobiocin against methicillin-resistant Staphylococcus aureus.

As infections due to methicillin-resistant Staphylococcus aureus become increasingly prevalent, newer alternative antibiotics, especially those which are orally administered, will be required. In order to provide an in-vitro basis for selecting alternative antibiotics, we studied the susceptibility of 103 strains of methicillin-resistant Staph. aureus from seven institutions to oral antimicrobial agents. Novobiocin, rifampicin, and trimethoprim-sulphamethoxazole had excellent in-vitro activity against virtually all strains of methicillin-resistant Staph. aureus. The MIC90 and MBC90 of novobiocin against methicillin-resistant Staph. aureus were 0.25 mg/l. Since previously reported achievable serum levels with oral novobiocin are 100 to 200 times its MIC90 against methicillin-resistant Staph. aureus, novobiocin should be evaluated further for combination therapy with rifampicin or trimethoprim-sulphamethoxazole.

Latamoxef in combination with aminoglycosides against Pseudomonas aeruginosa: similarity with ticarcillin.

The in vitro effect of latamoxef against 50 clinical strains of Pseudomonas aeruginosa was compared to that of ticarcillin, both alone and in combination with the aminoglycosides gentamicin, tobramycin and amikacin. Alone, the MIC90 of latamoxef was consistently one-half the MIC90 of ticarcillin. The two antibiotics appeared similar in regard to inoculum effect and bacterial killing. Adding of one-quarter the minimum inhibitory concentration of the aminoglycoside antibiotic to the beta lactam caused reduction in MIC90 of the latter (either ticarcillin or latamoxef) by one-half and decreased the MIC50 by almost one-quarter the concentration required by the beta lactams singly. Therefore, latamoxef singly or in combination with aminoglycosides behaved similarly but was more active than ticarcillin. Using combinations of antibiotics likely to be achieved in the serum of patients, a beneficial in vitro effect (either additive, partially synergistic or synergistic) generally occurred for the beta lactam-aminoglycoside combination if the strain was relatively sensitive to the aminoglycoside used in this combination. It occurred much less frequently for the highly aminoglycoside-resistant isolates.

The serum bactericidal activity of latamoxef (moxalactam), cefoperazone and cefotaxime.

We determined the serum bactericidal activity 1 h after the end of 2 g, 30 min infusions of latamoxef, cefoperazone and cefotaxime in six volunteers against six strains each of Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus and Pseudomonas aeruginosa. All produced excellent serum bactericidal activity against E. coli. Latamoxef and cefotaxime were superior for K. pneumoniae. Cefoperazone produced the highest titres against Staph. aureus. None of these agents produced sufficient bactericidal activity against Ps. aeruginosa to be useful in initial single agent therapy for the septic, granulocytopenic cancer patient.

Postantibiotic effect of imipenem on Pseudomonas aeruginosa.

Imipenem (formerly N-formimidoyl thienamycin) and ceftazidime were investigated for their postantibiotic effect on Pseudomonas aeruginosa. Four strains of P. aeruginosa in the logarithmic phase of growth were exposed for 1 and 2 h to concentrations of antibiotics achievable in human serum. Recovery periods of test cultures were evaluated after dilution or addition of beta-lactamase. A consistent postantibiotic effect against all strains was obtained with imipenem but not with ceftazidime. Although ceftazidime did not have a postantibiotic effect, it did suppress the growth of the organisms at concentrations equivalent to one-third of the MIC. The clinical implications of these effects need further evaluation.

Prophylaxis against enterococcal endocarditis: comparison of the aminoglycoside component of parenteral antimicrobial regimens.

Prophylactic antibiotics for the prevention of enterococcal endocarditis are recommended for patients with valvular heart disease undergoing surgery or instrumentation of the genitourinary and gastrointestinal tracts. To evaluate the most active aminoglycoside antibiotic to include in these regimens, we administered streptomycin, gentamicin, or amikacin, each in combination with ampicillin, to six healthy adult volunteers in a crossover manner. When the sera from the volunteers were tested for bactericidal activity against 16 strains of enterococci, the gentamicin-ampicillin combination produced higher serum bactericidal levels for a longer duration of time against more strains than the other two regimens. At 1 h after antibiotic administration (a time when surgical procedures are likely to be performed), mean geometric bactericidal titers against the enterococci were 1: 7.0 for the gentamicin-ampicillin regimen, as compared with 1:3.6 and 1:3.2 for the streptomycin-ampicillin and amikacin-ampicillin combinations, respectively. Despite the lower serum levels for gentamicin, we feel that this aminoglycoside should be used in combination with ampicillin for prophylactic regimens against enterococcal endocarditis.

Gentamicin and ticarcillin serum levels.

Ticarcillin disodium and gentamicin sulfate serum levels were measured one and five hours after intravenous administration. Patients receiving ticarcillin plus gentamicin had a significantly lower mean gentamicin level one and five hours after antibiotic administration than patients receiving cephalothin sodium plus gentamicin. The serum ticarcillin levels were significantly lower five hours after administration in patients receiving ticarcillin plus gentamicin than in those receiving ticarcillin plus cephalothin. Low antibiotic serum levels in patients with serious infections treated with these antibiotic combinations are of potential clinical significance; therefore, to ensure an optimal antimicrobial therapy, it is advisable to determine the drug serum concentrations (especially aminoglycoside) even in patients with normal renal function when treating with combinations of antimicrobials.

Parenteral prophylaxis against enterococcal endocarditis.

Three commonly used antibiotic regimens for the prevention of enterococcal endocarditis were administered parenterally to six healthy men in a crossover manner. The regimens included 1 gm of streptomycin intramuscularly (IM) in combination with (1) procaine penicillin 600,000 units plus aqueous penicillin G 200,000 units IM; or (2) ampicillin 25 mg/kg intravenously (IV); or (3) ampicillin 1 gm IM. The combinations containing ampicillin IM or IV with streptomycin produced bactericidal activity at dilutions of 1:2 or greater for the majority of the strains, whereas the penicillin-streptomycin regimen did not. All regimens were poorly bactericidal against three strains of enterococci which were highly resistant to streptomycin. These data suggest that ampicillin plus streptomycin is the preferred regimen for prophylaxis.

Comparison of serum bactericidal activity among three antimicrobial combinations.

Three antimicrobial combinations, ticarcillin plus cephalothin (T+C), ticarcillin plus gentamicin (T+G), and cephalothin plus gentamicin (C+G), were administered to 105 febrile granulocytopenic cancer patients at the Baltimore Cancer Research Center as part of a multi-institutional prospective randomized antibiotic trial. The sera from 32 of these patients (T+C-10 patients, T+G-10 patients, and C+G-12 patients) obtained 1 h post-antibiotic administration were examined for bactericidal activity against 11 strains each of the most common pathogens infecting the granulocytopenic host: Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, and Staphylococcus aureus. Each of the three antibiotic regimens produced a high degree of bactericidal activity in these sera against S. aureus and E. coli. P. aeruginosa was equally, although poorly, killed by sera containing ticarcillin (T+G, T+C), whereas C+G produced no measurable serocidal activity (P < 0.05). Sera with C+G killed K. pneumoniae more effectively than T+G; T+C produced the least killing effect of the three regimens against this organism (P < 0.05). The bactericidal activity of the serum from these 32 patients supplements the overall clinical results of the multi-institutional antibiotic trial and suggests that T+G is a useful initial regimen for empiric therapy of febrile episodes in granulocytopenic cancer patients.

Pirbenicillin: comparison with carbenicillin and BL-P1654, alone and with gentamicin, against Pseudomonas aeruginosa.

Minimum inhibitory concentrations (MIC) of pirbenicillin against 135 clinical isolates of Pseudomonas aeruginosa were one-fourth of those required for carbenicillin but two times higher than those for BL-P1654. Increasing the inoculum size produced an adverse effect on the bactericidal activity for all three antibiotics. This was more apparent for pirbenicillin than for carbenicillin, but less than the effect on BL-P1654. When concentrations of antibiotics likely to be achieved clinically were used, gentamicin increased the inhibitory and bactericidal effects of all three semisynthetic penicillins for the majority of isolates. Strains highly resistant to the aminoglycoside antibiotic, however, were inhibited no more by the penicillin-gentamicin combinations than by the most effective of the antibiotics alone.

BL-P1654, ticarcillin, and carbenicillin: in vitro comparison alone and in combination with gentamicin against Pseudomonas aeruginosa.

Minimum inhibitory concentrations of carbenicillin, ticarcillin, and BL-P1654 were determined for 89 clinical isolates of Pseudomonas aeruginosa. Ticarcillin was generally twice as active and BL-P1654 eight to 16 times as active as carbenicillin. Usually carbenicillin and ticarcillin killed at the same concentration or twice the concentration needed to inhibit, whereas 400 mug of BL-P1654 per ml was not bactericidal for the majority of isolates tested. The inhibitory effect of all three drugs varied markedly with the size of bacterial inoculum. When therapeutically achievable concentrations were used, adding gentamicin enhanced the inhibitory and bactericidal activity of all three penicillin derivatives for the majority of isolates. However, inhibition of isolates highly resistant to gentamicin was not improved by combining the semisynthetic penicillins with gentamicin.