Technical aspects and clinical correlations of the serum bactericidal test

Pharmacokinetics of sparfloxacin and serum bactericidal activity against pneumococci

Sparfloxacin, a new fluorinated quinolone, exhibits higher in vitro activity against pneumococci than do ciprofloxacin and ofloxacin. Since up to 30% of cases of pneumococcal pneumonia are associated with bacteremia, and since an increasing percentage of pneumococci are resistant against penicillin, we studied the serum bactericidal activity of sparfloxacin against pneumococci in eight healthy, middle-aged volunteers. Pharmacokinetics in serum and urine after a 400-mg oral dose of sparfloxacin were comparable to those described by other authors. Inhibitory and bactericidal activities in serum were measured for four pneumococcal isolates representing penicillin-susceptible (one isolate), intermediately resistant (two isolates), and highly resistant (one isolate) strains. Geometric mean inhibitory titers ranged between 1:2.4 and 1:6.3 and bactericidal titers ranged between 1:1.3 and 1:3.6 during a time period of 1 to 6 h after drug intake. Although such titers were not sufficient to predict a clinical response based on previous pharmacodynamic studies using quinolone antibiotics, data obtained with volunteers may only partially reflect the clinical situation in which a rise of humoral antibodies directed against pneumococcal antigens may help to reinforce the bactericidal action of the antibiotic.

Comparative serum bactericidal activities of three doses of ciprofloxacin administered intravenously

The pharmacokinetics and serum bactericidal activities of three intravenous doses of ciprofloxacin were studied comparatively in 30 patients. Single 200-, 300-, and 400-mg intravenous doses of ciprofloxacin were given over 30 min to 10 patients each, and serum samples were obtained at 0.5, 1, 2, 3, 4, 8, and 12 h after the start of the infusion. Serum drug concentrations were determined by high-pressure liquid chromatography. Pharmacokinetic parameters were estimated by using noncompartmental analysis methods. Serum bactericidal activity against clinical isolates of Escherichia coli, Enterobacter cloacae, Pseudomonas aeruginosa, Acinetobacter calcoaceticus, and Staphylococcus aureus was determined for samples obtained at 0.5, 4, 8, and 12 h. Excellent activity was demonstrated up to 12 h by all doses against E. coli and E. cloacae. Much poorer titers were observed for the remaining organisms, although the 400-mg dose prompted improved results against P. aeruginosa with a mean bactericidal titer of 1:2.9 at 8 h. In conclusion, while the 200-mg dose appears to be largely adequate for infections caused by members of the family Enterobacteriaceae, it seems that when P. aeruginosa is involved, 400 mg twice a day or even three times a day is more appropriate. Intravenous ciprofloxacin performs poorly against A. calcoaceticus and S. aureus, even at a higher dose.

MIC and serum bactericidal activity of clindamycin against methicillin-resistant and -sensitive staphylococci

Six volunteers were given 600 mg clindamycin intravenously to investigate the serum bactericidal activity (SBA) against 50 methicillin susceptible (MSSA) and 50 methicillin resistant Staphylococcus aureus (MRSA) strains. Minimal inhibitory concentrations (MIC) against MSSA, MRSA and 50 methicillin resistant strains of Staphylococcus epidermidis (MRSE), of which 50% were slime-producing, were determined. SBA of clindamycin against MSSA and MRSA was equally high (mean reciprocal SBA titer against MSSA vs MRSA 1 h after application was 13.0 vs 13.45), although MICs against MRSA were markedly higher than against MSSA (MIC 90 of MRSA vs MSSA: 0.06 vs > 32 mg/l). There was no difference in MICs between slime- and non-slime-producing MRSE.

Serum bactericidal activity after administration of four cephalosporins in healthy volunteers

Serum bactericidal activity (SBA) was determined against ten strains each of Staphylococcus aureus, Klebsiella pneumoniae, Proteus vulgaris and Enterobacter cloacae in six volunteers 1 h and 4 h after intravenous infusion of 1 g and 2 g cefotaxime and cefmenoxime, and 2 g flomoxef, and against ten strains of Pseudomonas aeruginosa after infusion of 1 g and 2 g ceftazidime. Flomoxef showed the highest SBA against methicillin-susceptible Staphylococcus aureus. All cephalosporins had high SBA against gram-negative rods. Cefotaxime had the highest SBA against Klebsiella pneumoniae and Enterobacter cloacae. The SBA against Pseudomonas aeruginosa after 1 g and 2 g doses of ceftazidime was very similar.

Ex vivo pharmacodynamic study of piperacillin alone and in combination with tazobactam, compared with ticarcillin plus clavulanic acid

Ten volunteers received piperacillin (4 g), piperacillin (4 g) plus tazobactam (0.5 g) (Tazocin), and ticarcillin (3 g) plus clavulanic acid (0.2 g) (Timentin) intravenously over 30 min in a cross-over blinded scheme. Blood samples were obtained 0.5 and 3 h after the end of infusion to measure by (high-pressure liquid chromatography) the concentration and bactericidal titers against 70 gram-negative bacilli. Serum time-kill curves were done against 35 strains to measure killing rates and area under the time-kill curve. Using the measure of serum bactericidal activity, ticarcillin-clavulanic acid and piperacillin-tazobactam were equally effective against Pseudomonas aeruginosa, Escherichia coli, Enterobacter cloacae, Serratia marcescens, and Bacteroides fragilis. Piperacillin-tazobactam was superior to ticarcillin-clavulanic acid against piperacillin-resistant Klebsiella pneumoniae (4 to 16 times) and S. marcescens (2 to 4 times). By using the area under the time-kill curve, piperacillin-tazobactam was equivalent to ticarcillin-clavulanic acid against piperacillin-susceptible strains; piperacillin-tazobactam was significantly more active than piperacillin against piperacillin-resistant strains and was more active than ticarcillin-clavulanic acid when the sample obtained 3 h after the end of infusion to volunteers was considered. Serum piperacillin concentrations (mean +/- standard error of the mean; in mg/liter) were 115 +/- 13 at 0.5 h and 7.4 +/- 1.4 at 3 h after the administration of piperacillin alone and 105.5 +/- 12.6 (0.5 h) and 7.7 +/- 1.6 after the administration of piperacillin-tazobactam. Serum tazobactam concentrations (in milligram per liter) were 13.1 +/- 1.4 at 0.5 h and 1.2 +/- 0.2 at 3 h. The piperacillin-tazobactam ratio was 8 +/- 0.3 at 0.5 h and 6.2 +/- 0.5 at 3 h. Piperacillin-tazobactam appears promising against beta-lactamase-producing gram-negative bacilli.

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.

Bactericidal activity of ciprofloxacin compared with that of cefotaxime in normal volunteers

We compared ciprofloxacin (200 mg) with cefotaxime (2 g) when each was administered intravenously over a 30-min period to six volunteers in a crossover manner 1 week apart. To integrate the pharmacologic and microbiologic activity, inhibitory and bactericidal activities in serum were obtained for both antibiotics 1 and 6 h after administration against 10 strains of Escherichia coli, 10 strains of Klebsiella pneumoniae, 15 strains of Pseudomonas aeruginosa, and 10 strains each of methicillin-susceptible and -resistant Staphylococcus aureus. Geometric mean bactericidal titers for E. coli 1 h after the infusion were 1:60 for ciprofloxacin and 1:252 for cefotaxime, and for K. pneumoniae they were 1:20 and 1:256, respectively. However, geometric mean titers were poor for both antibiotics against methicillin-susceptible S. aureus (less than 1:2 for ciprofloxacin versus 1:5 for cefotaxime) and methicillin-resistant S. aureus (less than 1:2 for both antibiotics), as well as against P. aeruginosa (1:3 for ciprofloxacin versus 1:2 for cefotaxime). These data suggest that ciprofloxacin may be useful for the treatment of serious infections caused by E. coli and K. pneumoniae. However, caution is suggested when this dose of ciprofloxacin is used in situations in which septicemia is caused by P. aeruginosa or S. aureus and originates outside the urinary tract.