Use of sodium polyanethol sulfonate to selectively inhibit aminoglycoside and polymyxin antibiotics in a rapid blood level antibiotic assay

SPS-neutralization in tissue samples for efficacy testing of antimicrobial peptides

Background

Accurate determination of the efficacy of antimicrobial agents requires neutralization of residual antimicrobial activity in the samples before microbiological assessment of the number of surviving bacteria. Sodium polyanethol sulfonate (SPS) is a known neutralizer for the antimicrobial activity of aminoglycosides and polymyxins. In this study, we evaluated the ability of SPS to neutralize residual antimicrobial activity of antimicrobial peptides [SAAP-148 and pexiganan; 1% (wt/v) in PBS], antibiotics [mupirocin (Bactroban) and fusidic acid (Fucidin) in ointments; 2% (wt/wt))] and disinfectants [2% (wt/wt) silver sulfadiazine cream (SSD) and 0.5% (v/v) chlorhexidine in 70% alcohol].

Methods

Homogenates of human skin models that had been exposed to various antimicrobial agents for 1 h were pipetted on top of Methicillin-resistant Staphylococcus aureus (MRSA) on agar plates to determine whether the antimicrobial agents display residual activity.

To determine the optimal concentration of SPS for neutralization, antimicrobial agents were mixed with PBS or increasing doses of SPS in PBS (0.05–1% wt/v) and then 10⁵ colony forming units (CFU)/mL MRSA were added. After 30 min incubation, the number of viable bacteria was assessed. Next, the in vitro efficacy of SAAP-148 against various gram-positive and gram-negative bacteria was determined using PBS or 0.05% (wt/v) SPS immediately after 30 min incubation of the mixture. Additionally, ex vivo excision wound models were inoculated with 10⁵ CFU MRSA for 1 h and exposed to SAAP-148, pexiganan, chlorhexidine or PBS for 1 h. Subsequently, samples were homogenized in PBS or 0.05% (wt/v) SPS and the number of viable bacteria was assessed.

Results

All tested antimicrobials displayed residual activity in tissue samples, resulting in a lower recovery of surviving bacteria on agar. SPS concentrations at ≥0.05% (wt/v) were able to neutralize the antimicrobial activity of SAAP-148, pexiganan and chlorhexidine, but not of SSD, Bactroban and Fucidin. Finally, SPS-neutralization in in vitro and ex vivo efficacy tests of SAAP-148, pexiganan and chlorhexidine against gram-positive and gram-negative bacteria resulted in significantly higher numbers of CFU compared to control samples without SPS-neutralization.

Conclusions

SPS was successfully used to neutralize residual activity of SAAP-148, pexiganan and chlorhexidine and this prevented an overestimation of their efficacy.

Contribution of the BacT/Alert MB Mycobacterium bottle to bloodstream infection surveillance in Thailand: added yield for Burkholderia pseudomallei

Community-acquired bloodstream infections cause substantial morbidity and mortality worldwide, but microbiology capacity and surveillance limitations have challenged good descriptions of pathogen distribution in many regions, including Southeast Asia. Active surveillance for bloodstream infections has been conducted in two rural Thailand provinces for >7 years. Blood specimens were divided into two culture bottles, one optimized for aerobic growth (F bottle) and a second for enhanced growth of mycobacteria (MB bottle), and processed with the BactT/Alert 3D system. Because the routine use of MB culture bottles is resource intensive (expensive and requires prolonged incubation), we assessed the added yield of MB bottles by comparing the proportion of pathogens detected by MB versus that by F bottles from 2005 to 2012. Of 63,066 blood cultures, 7,296 (12%) were positive for at least one pathogen; the most common pathogens were Escherichia coli (28%), Burkholderia pseudomallei (11%), Klebsiella pneumoniae (9%), and Staphylococcus aureus (6%). Two bottles improved the yield overall, but the added yield attributable to the MB bottles was limited to a few pathogens. In addition to the detection of mycobacteria and some fungi, MB bottles improved the detection of B. pseudomallei (27% [MB] versus 8% [F]; P < 0.0001), with added benefit if therapy was initiated prior to the blood culture. The targeted use of MB bottles is warranted for patients at risk for mycobacterial and fungal infections and for infection with B. pseudomallei, a common cause of septicemia in Thailand.

Controlled clinical comparison of BacT/ALERT standard aerobic and standard anaerobic blood culture bottles inoculated directly or after transport in sodium polyanethol sulfonate tubes

To assess relative performances in the BacT/ALERT blood culture system, we compared results from the direct inoculation of standard media and inoculation after the transport of blood samples in Vacutainer tubes with sodium polyanethol sulfonate. No significant differences in yields or times to detection were found for 387 clinically important isolates from 4,306 blood culture sets.

Pharmacokinetic/pharmacodynamic assessment of the in-vivo efficacy of imipenem alone or in combination with amikacin for the treatment of experimental multiresistant Acinetobacter baumannii pneumonia

A guinea-pig pneumonia model involving imipenem-susceptible and imipenem-resistant strains of Acinetobacter baumannii was developed to assess the in-vitro and in-vivo activities of imipenem, alone or in combination with amikacin, and the pharmacokinetic and pharmacodynamic parameters. Serum levels were measured by bioassay (imipenem) or immunoassay (amikacin), followed by calculation of pharmacokinetic and pharmacodynamic parameters (Cmax, AUC, t1/2, Cmax/MIC, AUC/MIC, and Deltat/MIC). In-vivo efficacy was evaluated by comparing bacterial counts in the lungs of treatment groups with end-of-therapy controls by anova and post-hoc tests. Decreases in the Cmax (13.4%), AUC (13%), t1/2 (25%) and Deltat/MIC (11.8-32.2%) of imipenem were observed when it was administered with amikacin, compared with administration of imipenem alone. Similarly, decreases in the Cmax (34.5%), AUC (11.6%), Cmax/MIC (34.5%) and AUC/MIC (11.7%) of amikacin were observed when it was administered with imipenem. Bacterial counts in lungs were reduced by imipenem (p 0.004) with the imipenem-susceptible strain, and by amikacin (p 0.001) with the imipenem-resistant strain. The combination of imipenem plus amikacin was inferior to imipenem alone with the imipenem-susceptible strain (p 0.01), despite their in-vitro synergy, and was inferior to amikacin alone with the imipenem-resistant strain (p < 0.0001). In summary, combined use of imipenem with amikacin was less efficacious than monotherapy, probably because of a drug-drug interaction that resulted in decreased pharmacokinetic and pharmacodynamic parameters for both antimicrobial agents.

Performance of a BACTEC nonradiometric medium for pediatric blood cultures

A BACTEC aerobic nonradiometric medium, PEDS Plus, designed for diagnosis of pediatric bacteremia was evaluated in three hospital centers. Equivalent blood volumes (up to 5 ml) were inoculated into and incubated in BACTEC NR-6A (6A) and PEDS Plus broths. Among 4,581 compliant sets, 289 clinically significant organisms, representing more than 20 bacterial and two Candida species, were isolated. One hundred eighty-one isolates were recovered in both bottles, 75 in PEDS Plus only, and 33 in 6A only (P less than 0.001). Time to detection when both bottles were positive was the same for 129 isolates, detection with PEDS Plus was earlier for 39, and detection with 6A was earlier for 13 (P less than 0.005). Staphylococcus aureus was recovered significantly more often in PEDS Plus than in 6A (P less than 0.01), and more coagulase-negative staphylococci and pediatric pathogens (pneumococci, Haemophilus influenzae, and Streptococcus agalactiae) were recovered in PEDS Plus than in 6A. Coagulase-negative staphylococci and H. influenzae were detected significantly earlier in PEDS Plus (P less than 0.05 and less than 0.01, respectively). When the eight species of the family Enterobacteriaceae isolated were considered together, recovery in PEDS Plus was better than in 6A (P less than 0.05). For 66 of the 143 isolates from patients known to be on antimicrobial therapy at the time blood was drawn, PEDS Plus was superior to 6A. In 45 cases, organisms were isolated from PEDS Plus only (P less than 0.001) and in 21 cases they were isolated from PEDS Plus before 6A (P less than 0.01). PEDS Plus broth aids diagnosis of pediatric bacteremia by increasing recovery of etiologic agents and decreasing the time required for detection.

Ex vivo study of serum bactericidal titers and killing rates of daptomycin (LY146032) combined or not combined with amikacin compared with those of vancomycin

Twelve volunteers, in two groups of six, received daptomycin at a dose of 1 or 2 mg/kg. In addition, they received in a randomly allocated order amikacin (500 mg), daptomycin-amikacin, and vancomycin (500 mg). Thirty-five clinical isolates, including Staphylococcus aureus, S. epidermidis, Corynebacterium sp. group JK, and Enterococcus faecalis, were tested in vitro for the measure of the serum bactericidal titers and killing rates. The mean peak concentrations of daptomycin in serum 1 h after the administration of 1 and 2 mg/kg were 11 and 20 micrograms/ml, respectively. At 24 h after the administration of 2 mg/kg, the mean level in serum was 1.9 micrograms/ml, which is higher than the MICs for susceptible pathogens. Daptomycin and amikacin provided identical concentrations in serum whether given alone or in combination. Among the six regimens tested, those including daptomycin provided the highest and the most prolonged serum bactericidal titers against S. aureus, S. epidermidis, and E. faecalis. The killing rates measured by the killing curves were correlated with the concentration/MIC and concentration/MBC ratios of daptomycin for all strains tested. Significant killing occurred once the concentration of daptomycin in the serum 4- to 6-fold the MIC or 1- to 1.2-fold the MBC. The combination of daptomycin and amikacin had no effect on either the serum bactericidal titers or the rates of killing. Only vancomycin provided significant killing of the strains of Corynebacterium sp. group JK.

Serum bactericidal activity and killing rate for volunteers receiving imipenem, imipenem plus amikacin, and ceftazidime plus amikacin against Pseudomonas aeruginosa

Serum bactericidal activity against 20 strains of Pseudomonas aeruginosa was studied in 10 volunteers after administration of imipenem (25 mg/kg), imipenem (25 mg/kg) plus amikacin (7.5 mg/kg), and ceftazidime (25 mg/kg) plus amikacin (7.5 mg/kg). Eight strains were susceptible and 12 were resistant to ticarcillin. Serum levels were measured microbiologically after 30 and 60 min and were, respectively, 97 and 46 micrograms/ml for imipenem given alone and 79 and 45 micrograms/ml for imipenem given with amikacin. Despite the very large dose of imipenem used, imipenem and imipenem plus amikacin appeared slightly less active than ceftazidime plus amikacin (P less than or equal to 0.1; Wilcoxon matched-pairs test), with respective median titers at 30 min of 1:128, 1:128, and 1:256 against ticarcillin-susceptible strains and 1:32, 1:32, and 1:64 against ticarcillin-resistant strains; however, more than 90% of the serum determinations, regardless of the regimen, had a serum bactericidal activity greater than or equal to 1:8. Amikacin significantly increased the rate of killing in serum of P. aeruginosa by imipenem. Imipenem plus amikacin appeared as effective as ceftazidime plus amikacin in reducing the viable counts of P. aeruginosa after 24 h of incubation.

Bactericidal activity and killing rate of serum from volunteers receiving pefloxacin alone or in combination with amikacin

Serum bactericidal activities (SBAs) were studied after intravenous administration of pefloxacin (8 mg/kg) and amikacin (7.5 mg/kg) alone or in combination to 15 human volunteers. About 10 strains each of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus were tested. The serum levels of pefloxacin were measured microbiologically by using E. coli KP 1976-712 as the test organism at 0, 30, 60, 120, and 720 min after infusion; at 0, 30, 60, and 720 min these levels were 7 +/- 1.4, 5 +/- 0.8, 4.5 +/- 0.7, and 2.1 +/- 0.6 mg/liter (mean +/- standard deviation), respectively, with a terminal half-life of 10 h. The serum levels of pefloxacin in the presence of amikacin were measured similarly; 1% sodium polyanethol sulfonate was added to the agar to inactivate amikacin. Treatment with pefloxacin alone resulted in high SBAs against E. coli, K. pneumoniae strains susceptible to cephalothin, and Staphylococcus aureus at the peak concentration; 81 to 100% of the sera had SBAs of greater than or equal to 1:8. However, treatment with pefloxacin alone resulted in low SBAs against K. pneumoniae strains resistant to cephalothin and P. aeruginosa; only 34% of the sera had SBAs of greater than or equal to 1:8. At trough concentrations the percentages of sera with SBAs greater than or equal to 1:8 were 75 to 83% (E. coli), 9 to 27% (K. pneumoniae), 0% (P. aeruginosa), and 10% (S. aureus). The combination of pefloxacin plus amikacin was most often additive; the peak activity was due to amikacin, and the trough activity was due to pefloxacin. Occasionally antagonism occurred with P. aeruginosa, K. pneumoniae, and S. aureus strains. These observations were confirmed by the killing curves in pooled serum obtained at peak and trough levels. Regrowth was observed for seven strains of P. aeruginosa treated with pefloxacin alone; amikacin seemed to prevent this phenomenon.

Effect of NaCl supplementation of Mueller-Hinton broth on susceptibility of staphylococci to aminoglycosides

The addition of NaCl to cation-supplemented Mueller-Hinton broth increased the MICs of gentamicin, amikacin, and netilmicin for coagulase-positive and -negative staphylococci in a concentration-dependent manner. At 2% NaCl, geometric mean MICs were elevated 14.1- to 25.6-fold. Mueller-Hinton broth containing added NaCl should not be used for testing the susceptibility of staphylococci to aminoglycosides.

Serum bactericidal activity of aztreonam, cefoperazone, and amikacin, alone or in combination, against Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, and Pseudomonas aeruginosa

Serum samples from volunteers receiving (per kilogram) 20 mg of aztreonam, 20 mg of cefoperazone, 7.5 mg of amikacin, 20 mg of cefoperazone plus 20 mg of aztreonam, or 20 mg of aztreonam plus 7.5 mg of amikacin were evaluated for bactericidal activity against Escherichia coli, Klebsiella pneumonia, Serratia marcescens, and Pseudomonas aeruginosa. Serum bactericidal activities were similar for aztreonam alone or in combination but were lower for amikacin and cefoperazone alone, especially against S. marcescens and P. aeruginosa. Killing studies, performed with serum samples diluted 1:8, demonstrated a high killing rate for aztreonam plus amikacin, especially against P. aeruginosa.

Developmental pharmacokinetics of mezlocillin in newborn infants

Single-dose pharmacokinetics of mezlocillin were studied in 53 newborn infants (72% less than or equal to 36 weeks' gestation) given ampicillin and an aminoglycoside for suspected or proved sepsis. Mezlocillin (75 mg/kg IV or IM) was substituted for ampicillin, serum was assayed microbiologically, and noncompartmental pharmacokinetic parameters were calculated. Analysis of covariance showed that dose/area under the serum concentration-time curve for mezlocillin was influenced by body weight, intramuscular administration, and treatment with gentamicin. A dual intravenous/intramuscular nonlinear regression model yielded an apparent intramuscular bioavailability of 84%. Clearance was proportional to body weight (WT) (r2 = 0.70). Mean CL/WT (0.078 L/hr/kg) was one-half adult values and influenced by gestational age. Steady-state volume of distribution varied linearly with weight (r2 = 0.80), the mean value (0.38 L/kg) being twice that in adults. Mezlocillin half-life (mean 3.71 hours) exceeded adult values and did not correlate with weight. Twenty-four newborn infants received 75 mg/kg mezlocillin every 6 or every 8 hours, along with gentamicin, during the first 7 to 10 days of life. Peak (1.5 hours) and trough (6 or 8 hours) concentrations were determined; the latter decreased from day 3 to days 7 to 10, suggesting a possible postnatal age-dependent change in mezlocillin elimination. Although mezlocillin disposition is affected by age and therapeutic factors, weight alone may adequately predict dosing requirements.

Inactivation of the polyanionic detergent sodium polyanetholsulfonate by hemoglobin

Sodium polyanetholsulfonate (SPS) has been added to blood culture media for many years. Its incorporation results in a higher yield of positive blood cultures due to its inactivation of antimicrobial cationic compounds. The most active of these cations include complement components, aminoglycoside-aminocyclitol antibiotics, and receptors on polymorphonuclear leukocytes. There have been reports from studies conducted outside patient blood culture bottles that SPS itself may possess antibacterial activity against some isolates of Neisseria meningitidis, Neisseria gonorrhoeae, and Peptostreptococcus anaerobius. Conversely, in patient clinical trials there has been no significant difference in pathogen isolation rates in the presence or absence of SPS. In an attempt to explain this in vitro/in vivo disparity, a search was undertaken to elucidate which variable constituent in blood, heretofore not studied quantitatively, might have a major effect on modulating the activity of SPS. It was found that hemoglobin combined stoichiometrically with SPS with a Kd of approximately 10(-7) mol/liter. Optimum SPS inactivation occurred at an SPS/hemoglobin ratio of 1:6 (wt/wt). SPS-sensitive isolates of N. gonorrhoeae and N. meningitidis were protected by the addition of hemoglobin from the antimicrobial effects of this polyanion in time-kill studies. This protection was directly related to the amount of SPS combined in solution. Therefore, the amount of free hemoglobin in solution must be measured when studying the antimicrobial activity of polyanions or when evaluating the effect of different polyanions on the recovery rates of pathogens in patient blood culture clinical trials.

Disinfection of human heart valve allografts with antibiotics in low concentration

Antibiotic disinfection of allograft heart valves is now an established procedure. A new low-concentration, broad-spectrum mixture of antibiotics was formulated and used successfully to disinfect 58 of 60 human aortic and pulmonary valves after 48 h exposure at 4 degrees C. The antibiotics included were cefoxitin 240 mcg/ml, lincomycin 120 mcg/ml, polymyxin B 100 mcg/ml, vancomycin 50 mcg/ml and amphotericin B 25 mcg/ml. Microorganisms isolated from experimental tissue and from clinical valves before treatment were tested for sensitivity to these antibiotics and were found to be sensitive to one or more. One isolate of Pseudomonas spp. showed decreased sensitivity to polymyxin B, and was found to have a minimum bactericidal concentration of 70 mcg/ml. Results indicated this antibiotic mixture had superior antibacterial efficiency to that in routine clinical use for the past 13 yr.

Pharmacokinetics of piperacillin and gentamicin following intravenous administration to dogs

Piperacillin sodium was administered intravenously to dogs, alone or in combination with gentamicin, twice a day (approximately 5 hr apart) for 36-37 days. The pharmacokinetics of neither drug changed in the presence of the other; however, the percentage of the gentamicin dose recovered in the urine decreased significantly when coadministered with piperacillin. The data demonstrate that interaction between the two drugs in urine is feasible.

Evaluation of sodium polyanethanol sulfonate as a blood culture additive for recovery of Salmonella typhi and Salmonella paratyphi A

A total of 640 blood specimens from patients in an area endemic for enteric fever were cultured in parallel in tryptic soy broth with and without sodium polyanethanol sulfonate (SPS). A total of 95 specimens were positive for Salmonella spp., 54 for Salmonella typhi, and 41 for Salmonella paratyphi A in one or both bottles of a set. Significantly higher rates of recovery were obtained from the SPS-containing medium (P less than 0.01) upon subculturing blindly at 24 h and 3 days of incubation. Subcultures performed at 7, 14, and 21 days also yielded a greater number of positive cultures with SPS than without it, although the differences between the two media were not significant (P greater than 0.05). Neither of the media yielded 100% of the positive cultures. Moreover, even if the results of the two media were combined, 34 and 19% of the isolates would have been missed if the specimens had not been incubated to 14 and 21 days, respectively. The data indicate that SPS aids in early recovery of S. typhi and S. paratyphi A from blood cultures, and additionally, that under the conditions used in the study, incubation beyond a 1-week period is required for efficient isolation of these organisms from blood.

Serum bactericidal activity of moxalactam and cefotaxime with and without tobramycin against Pseudomonas aeruginosa and Staphylococcus aureus

Serum bactericidal activity was determined against 10 strains each of Pseudomonas aeruginosa and Staphylococcus aureus in serum from volunteers 1 and 6 h after intravenous infusion of cefotaxime, tobramycin, and the combination; or of moxalactam, tobramycin, and the combination. High serum bactericidal activity against P. aeruginosa was found significantly more frequently with moxalactam plus tobramycin than with cefotaxime, moxalactam, and tobramycin alone or with cefotaxime plus tobramycin.

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.

Antibiotic activity in vitro against methicillin-resistant Staphylococcus epidermidis and therapy of an experimental infection

Staphylococcus epidermidis is a major pathogen in early prosthetic valve endocarditis and cerebrospinal fluid shunt infections. Approximately 10 to 15% of hospital isolates are methicillin resistant. Ten clinically significant isolates of the latter were collected for antibiotic studies in vitro and in an experimental infection in animals. Time-kill studies of five strains showed gentamicin to be the single most effective antibiotic; however, dwarf colony variants emerged as survivors with two of these strains when challenged with gentamicin alone. The addition of a second antibiotic to gentamicin did not significantly improve the bactericidal rate but prevented the emergence of variant strains. A blood culture isolate of methicillin-resistant S. epidermidis combined with 5% hog gastric mucin was used to establish an experimental intraperitoneal infection in mice. Neither methicillin nor nafcillin treatment reduced mortality below that of untreated animals. Cephalothin treatment delayed early mortality but did not diminish overall mortality. Gentamicin was the most effective single antibiotic, and gentamicin in combination with vancomycin was the most effective regimen overall. The combination of rifampin plus vancomycin was as effective as gentamicin alone. The combinations of cephalothin or nafcillin with gentamicin and cephalothin with vancomycin demonstrated antagonism. The antagonism was not due to multiple injections or drug-drug inactivation.

Simple method for elimination of aminoglycosides from serum to permit bioassay of other antimicrobial agents

Cellulose phosphate powder can be added to serum to selectively bind aminoglycosides by ionic interaction without binding or inactivation of the penicillins, the cephalosporins, clindamycin, vancomycin, chloramphenicol, or trimethoprim. This simple, rapid technique permits the measurement of non-aminoglycoside antibiotics in the presence of aminoglycosides.