Susceptibility of enterococci to trimethoprim and trimethoprim-sulfamethoxazole.

The in vitro activities of trimethoprim (TMP), alone and in combination with sulfamethoxazole (SMX), against 131 clinical isolates of enterococci, 126 Streptococcus faecalis isolates, and 5 Streptococcus faecium isolates were determined by a broth microdilution method with Mueller-Hinton broth that was substantially free of inhibitory substances. The geometric mean MIC of TMP for strains of S. faecalis was 0.164 micrograms/ml (range, 0.03 to 8 micrograms/ml), with a geometric mean MBC of 0.298 micrograms/ml (range, 0.063 to 8 micrograms/ml). Although all strains were resistant to the sulfonamide alone, the inhibitory and bactericidal activities of TMP against strains of S. faecalis were markedly potentiated when TMP was combined in a fixed ratio of 1:19 with SMX; the geometric mean MIC of TMP was reduced to 0.016 micrograms/ml (range, 0.002 to 0.25 micrograms/ml), with a geometric mean MBC of 0.031 micrograms/ml (range, 0.004 to 0.25 micrograms/ml). The combination had no synergistic effect against strains of S. faecium; the geometric mean MICs and MBCs of both agents were ca. 0.06 micrograms/ml. The MBC/MIC ratios for TMP and TMP-SMX were less than or equal to 16 for all 131 strains. MICs and MBCs for TMP-SMX were unchanged, and for TMP they decreased when performed in broth supplemented with 50% heat-inactivated pooled human serum. For TMP and TMP-SMX, the susceptibilities of isolates with high-level resistance to gentamicin or streptomycin were the same as those of isolates susceptible to less than or equal to 2,000 micrograms of aminoglycoside per ml. These results suggest that TMP-SMX and TMP alone could prove useful in the treatment of serious enterococcal infections, including infections by strains with high-level resistance to aminoglycosides.     

In vitro activity of RP 59500, a semisynthetic injectable pristinamycin, against staphylococci, streptococci, and enterococci.

The in vitro activity of RP 59500, a semisynthetic pristinamycin, was compared with the activities of vancomycin, oxacillin, ampicillin, gentamicin, ciprofloxacin, and rifampin against five Staphylococcus species, five Streptococcus species, and four Enterococcus species. For staphylococci, MICs were 0.13 to 1 microgram/ml and the MICs for 90% of the strains tested (MIC90s) were 0.13 to 0.5 microgram/ml; there were no differences between oxacillin-susceptible and -resistant strains. For streptococci, MICs were 0.03 to 4 micrograms/ml and MIC90s were 0.25 to 2 micrograms/ml; viridans group streptococci were the least susceptible streptococci. For enterococci, MICs were 0.25 to 32 micrograms/ml and MIC90s were 2 to 4 micrograms/ml; Enterococcus faecalis was the least susceptible. Vancomycin was the only comparative drug with consistent activity against all species of gram-positive cocci. With RP 59500, raising the inoculum 100-fold, lowering the pH of cation-adjusted Mueller-Hinton broth to 5.5, or omitting cation supplementation had little effect on MICs, but 50% serum increased MICs 2 to 4 dilution steps. The differences between MBCs and MICs were greater for staphylococci and enterococci than for streptococci. Time-kill studies with 24 strains indicated that RP 59500 concentrations 2-, 4-, and 16-fold greater than the MICs usually killed bacteria of each species at similar rates; reductions in CFU per milliliter were less than those observed with oxacillin or vancomycin against staphylococci and less than those observed with ampicillin against enterococci. RP 59500 antagonized the bactericidal activities of oxacillin and gentamicin against Staphylococcus aureus ATCC 29213 and that of ampicillin against E. faecalis ATCC 29212. Against the latter, combination with gentamicin was indifferent. RP 59500 has a broad spectrum of in vitro activity against gram-positive cocci; combining it with other drugs is not advantageous.     

Antibacterial activity of norfloxacin.

Norfloxacin, a new quinoline derivative, was studied in vitro, and determinations of agar dilution minimal inhibitory concentrations (MICs) and broth dilution MICs and MBCs were made. Nalidixic acid and cinoxacin were used as comparative agents. Norfloxacin was found to be extremely active against all strains tested of Escherichia coli, Klebsiella spp., Proteus mirabilis, indole-positive Proteus spp. Serratia spp., Citrobacter spp., and Enterobacter spp., with MICs normally below 1 microgram/ml. It also was found to be highly active against Pseudomonas aeruginosa, Staphylococcus saprophyticus, and enterococci, which are all resistant to nalidixic acid and cinoxacin. The MICs for norfloxacin obtained by broth dilution were slightly higher than those obtained by agar dilution, whereas the reverse was true for nalidixic acid and cinoxacin. The MBCs of norfloxacin were only slightly higher than the MICs, even at high inocula. The in vitro activity of norfloxacin was not dependent on the inoculum size, whereas both the MICs and the MBCs of nalidixic acid increased markedly for many of the strains tested when the inoculum was increased in broth dilution from 10(3) to 10(6) colony-forming units per ml. Norfloxacin seems to be a promising antibacterial agent for the treatment of urinary tract infections, especially those caused by Pseudomonas spp. and other species today requiring the use of injectable antibiotics.     

Activity of LY146032 against Enterococci with and without high-level aminoglycoside resistance, including two penicillinase-producing strains.

We tested the activity of LY146032 (LY) against 57 strains of enterococci collected from Chile, Thailand, and the United States. Some of the strains were resistant to high levels of gentamicin or streptomycin (or both), and two produced beta-lactamase (Bla+). MICs of LY ranged from 0.5 to 8 micrograms/ml, and MBCs ranged from 1 to 64 micrograms/ml. In time-kill assays, a 2 to 3 log10 killing effect was observed with LY against two Bla+ strains of Streptococcus (Enterococcus) faecalis and against three strains that were highly resistant to streptomycin and gentamicin. Synergism was demonstrated with LY and streptomycin against a Bla+ strain lacking high-level streptomycin resistance. These in vitro results suggest that LY should be studied further for possible use in treatment of enterococcal infections.     

Bacteriostatic and bactericidal activities of gentamicin alone and in combination with clarithromycin against Mycobacterium avium.

The inhibitory activity of gentamicin against Mycobacterium avium depended on the pH of the medium, and the broth-determined MICs for 90% of strains were 5.0 micrograms/ml at pH 7.4, 9.5 micrograms/ml at pH 6.8, and greater than 16.0 micrograms/ml at pH 5.0. The MBCs were two- to eightfold higher than the MICs. The combined effect of gentamicin and clarithromycin was additive, and the MICs and MBCs of each drug were either the same as those in the single-drug tests or reduced twofold.     

Treatment of experimental endocarditis due to methicillin-susceptible or methicillin-resistant Staphylococcus aureus with trimethoprim-sulfamethoxazole and antibiotics that inhibit cell wall synthesis.

Using two strains of Staphylococcus aureus, one susceptible and one heterogeneously resistant to methicillin, for which MICs and MBCs of trimethoprim-sulfamethoxazole (TMP-SMX) were 0.06 and 0.06 micrograms/ml and 0.06 and 0.25 microgram/ml, respectively (concentrations are those of TMP), we studied the efficacies of TMP-SMX and cloxacillin, teicoplanin, and vancomycin for treatment of experimental staphylococcal endocarditis. Rabbits were treated with dosages of TMP-SMX selected to achieve concentrations in serum equivalent to that obtained in humans treated for Pneumocystis carinii pneumonia. The overall mortality rate of rabbits treated with TMP-SMX was 84% at day 3, not different from that of the control groups (P > 0.1). No sterile vegetations were observed to be present in control groups or in animals treated with TMP-SMX. However, 26, 60, and 75% of rabbits treated with teicoplanin, cloxacillin, and vancomycin, respectively, showed sterile vegetations. For methicillin-susceptible S. aureus (MSSA), the mean vegetation counts were not significantly different between the control group and the group treated with TMP-SMX (P > 0.1). For methicillin-resistant S. aureus (MRSA), treatment with TMP-SMX was more effective than no therapy, decreasing the number of organisms in vegetations (P < 0.01). For both strains, therapy with cloxacillin and therapy with teicoplanin or vancomycin were significantly more effective than therapy with TMP-SMX. Despite high concentrations of teicoplanin in serum which exceeded MBCs for staphylococci more than 50 times at the peak and 10 times at the trough, therapy with cloxacillin or vancomycin was superior to therapy with teicoplanin against both MSSA and MRSA. These data do not support the use of TMP-SMX in treatment of endocarditis and other severe staphylococcal infections with high bacterial counts.     

In vitro activity of DNA gyrase inhibitors, singly and in combination, against Mycobacterium avium complex.

The in vitro activities of the DNA gyrase inhibitors ciprofloxacin, coumermycin, and novobiocin against 31 clinical isolates of Mycobacterium avium complex were studied using a microdilution technique. Minimal inhibitory concentrations (MICs) were determined in 4 days using Middlebrook 7H9 broth, and minimal bactericidal concentrations (MBCs) were determined by subculturing to Middlebrook 7H10 agar. MICs were: ciprofloxacin, 0.5-greater than 16 (mean, 4.1) micrograms/ml; novobiocin, 4-greater than 128 (mean, 54.7) micrograms/ml; and coumermycin, 2-greater than 16 (mean, 17.5) micrograms/ml. MBCs were usually more than two dilution steps higher than MICs. Checkerboard studies failed to reveal synergistic or antagonistic inhibitory activity of DNA gyrase-A and DNA gyrase-B inhibitors in vitro.     

Comparative In Vitro Appraisal of Piperacillin, Including Its Activity Against Salmonella typhi

Piperacillin was evaluated in vitro against 711 clinical isolates of aerobic and anerobic gram-positive and gram-negative bacteria, including 76 isolates of Salmonella typhi. Piperacillin minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were compared with those of a range of β-lactam, aminoglycoside, and other antimicrobial agents, and inoculum size effects were considered. The relationship between dilution and disk diffusion tests was studied by regression analysis. In addition, piperacillin was assessed in combination with aminoglycoside and other β-lactam drugs. This investigation has confirmed the activity of piperacillin against a broad range of bacteria, including Pseudomonas, Enterobacteriaceae, Neisseria, β-lactamase-negative Haemophilus influenzae, and Staphylococcus aureus as well as enterococci, Bacteroides fragilis, and other anaerobes. All strains of Pseudomonas aeruginosa were inhibited by ≤32 μg/ml or less, demonstrating again the potential usefulness of piperacillin in the treatment of pseudomonal infections. S. typhi proved susceptible to piperacillin, all isolates being inhibited by 1 μg/ml. Inoculum size experiments showed that inocula of 10⁸ CFU resulted in MICs and MBCs appreciably higher than those resulting from inocula of 10⁶ CFU, and inocula of 10² CFU resulted in MICs and MBCs appreciably lower than those resulting from inocula of 10⁴ CFU. Piperacillin was active against all gentamicin-resistant pseudomonads tested, but not against gentamicin-resistant klebsiellas and enterobacters. Combinations of piperacillin with tobramycin and amikacin were consistently synergistic against Pseudomonas and Serratia isolates. Less consistent results were shown when piperacillin was combined with aminoglycosides or cephalothin against Klebsiella and indole-positive Proteus isolates, although synergy was observed in most cases. Occasional antagonistic reactions were encountered with piperacillin-cephalothin or piperacillin-tobramycin combinations against the latter isolates.     

In vitro activity of LY333328, an investigational glycopeptide antibiotic, against enterococci and staphylococci.

The in vitro activities of LY333328 were compared with those of vancomycin, teicoplanin, and quinupristin-dalfopristin (Synercid) against 219 strains of enterococci and staphylococci, including vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus. MICs and MBCs were determined by a microtiter dilution protocol. LY333328 demonstrated superior activity against vancomycin-resistant enterococci and was the only antibiotic which was bactericidal. Its potency was comparable or superior to those of other antibiotics tested against methicillin-resistant staphylococci.     

Comparative in vitro inhibitory and killing activity of cefpirome, ceftazidime, and cefotaxime against Pseudomonas aeruginosa, enterococci, Staphylococcus epidermidis, and methicillin-susceptible and -resistant and tolerant and nontolerant Staphylococcus aureus.

With a macrotube dilution method, MICs and MBCs were determined for three aminothiazolyl cephalosporins, cefpirome (HR 810), ceftazidime, and cefotaxime, against Pseudomonas aeruginosa, enterococci, Staphylococcus epidermidis, and methicillin-resistant, -susceptible, and -tolerant strains of Staphylococcus aureus. Comparatively, cefpirome was the most active agent against all gram-positive cocci, including enterococci and methicillin-resistant S. aureus, and was as active as ceftazidime against P. aeruginosa. MBCs of cefpirome were within two dilutions of the MICs for 91% of P. aeruginosa and 90% of gram-positive cocci strains tested, except methicillin-resistant S. aureus, for which the MBCs were within three dilutions for 90% of strains.     

In vitro activity of LY146032 against staphylococci, streptococci, and enterococci.

The in vitro activities of LY146032 and seven comparative antimicrobial agents against 14 species of staphylococci, streptococci, and enterococci were studied. MICs of LY146032 were less than or equal to 0.5 microgram/ml for all staphylococci, including oxacillin-resistant strains; less than or equal to 0.25 microgram/ml for all streptococci (except viridans group streptococci); and less than or equal to 4 micrograms/ml for all viridans group streptococci and enterococci. MICs were minimally affected by variations in inoculum size, and LY146032 was bactericidal against all species tested.     

Influence of the developmental state of valvular lesions on the antimicrobial activity of cefotaxime in experimental enterococcal infections.

Cefotaxime has little antimicrobial activity in vitro against most strains of enterococci, as measured by conventional MICs and MBCs. However, the MICs of cefotaxime against many enterococci are markedly reduced by the addition of serum to the test medium. To assess the relevance of this observation in vivo, we examined the efficacy of cefotaxime in experimental Streptococcus faecalis endocarditis. Since response to antimicrobial agents may vary with the degree of vegetation development, therapeutic efficacy was assessed both in rabbits with newly formed vegetations and in rabbits with well-developed endocardial lesions. Peak serum levels of cefotaxime (50.1 +/- 20.0 micrograms/ml) exceeded the MIC in medium supplemented with serum (4 micrograms/ml), but not in Mueller-Hinton broth alone (greater than 64 micrograms/ml). After 4 days of therapy, animals with newly formed lesions (therapy initiated 1 h after infection, transvalvular catheters removed) had lower mean vegetation bacterial titers than did untreated controls. Among animals with mature vegetations (therapy initiated 12 h after infection, catheters indwelling), the rate of mortality was significantly reduced by cefotaxime therapy. However, no difference in vegetation titers was observed. Thus, cefotaxime demonstrated antienterococcal activity within newly formed vegetations, but did not inhibit bacterial proliferation within well-established vegetations.     

In vitro susceptibility of group G streptococci to ten antimicrobial agents with broad gram-positive spectra

Minimal inhibitory and bactericidal concentrations (MICs, MBCs) of ten antibiotics with broad spectra against grampositive cocci were determined for 25 group G streptococci (GGS). Penicillin G, cefotaxime, and ampicillin were the most active bactericidal agents, with 100% of MBCs less than or equal to 0.6 micrograms/ml. Among the nonpenicillin, noncephalosporin agents, vancomycin had the lowest MBCs, with all strains killed by less than or equal to 2.5 micrograms/ml. Despite good in vitro activity against other streptococci, erythromycin, clindamycin, and chloramphenicol had disappointing bactericidal activity against GGS. Three strains were tolerant to erythromycin, while 15 strains were tolerant to clindamycin. Chloramphenicol was the least active inhibitory agent, with an MIC90 of 13.8 micrograms/ml; this agent was also the least active bactericidal agent against GGS. with no MBCs less than 2.5 micrograms/ml and 9 strains with MBCs greater than or equal to 20 micrograms/ml. There were no GGS strains tolerant to either penicillin G or vancomycin.     

The bactericidal activity of ampicillin, daptomycin, and vancomycin against ampicillin-resistant Enterococcus faecium.

Ampicillin, daptomycin, and vancomycin, alone and in combination with gentamicin, were examined for bactericidal effects on ampicillin-resistant Enterococcus faecium using broth dilution minimum inhibitory concentrations (MICs) and time-kill studies. We tested 12 ampicillin-resistant isolates and demonstrated the following MICs and MBCs, respectively: ampicillin, greater than or equal to 32 micrograms/ml and greater than 256 micrograms/ml; daptomycin, less than or equal to 4 micrograms/ml and less than or equal to 16 micrograms/ml; and vancomycin, less than or equal to 4 micrograms/ml and greater than 64 micrograms/ml. Time-kill studies demonstrated that daptomycin alone had marked activity against the ampicillin-resistant E. faecium and that the addition of gentamicin resulted in synergistic killing. In addition, ampicillin and vancomycin were not bactericidal for the ampicillin-resistant isolates without the addition of gentamicin. The present study supports the consideration of daptomycin alone or in combination with an aminoglycoside as an alternative therapy for ampicillin-resistant enterococci, although additional clinical experience is now necessary.     

In vitro activity of cefquinome, a new cephalosporin, compared with other cephalosporin antibiotics.

The in vitro activity of cefquinome, a new aminothiazolyl cephalosporin with a C-3 bicyclic pyridinium group, was compared with ceftazidime, cefpirome, and cefepime. Cefquinome inhibited members of the Enterobacteriaceae at less than or equal to 0.5 microgram/ml for Escherichia coli, Klebsiella pneumoniae, K. oxytoca, Citrobacter diversus, Salmonella Shigella, Proteus mirabilis, Morganella, and Providencia. Although most Citrobacter freundii and Enterobacter cloacae were inhibited by less than 2 micrograms/ml, some strains resistant to ceftazidime were resistant, [minimum inhibitory concentration (MIC) greater than 16 micrograms/ml]. Serratia marcescens were inhibited by less than 1 microgram/ml and Pseudomonas aeruginosa by 8 micrograms/ml similar to the activity of cefepime. The majority of Haemophilus influenzae and Neisseria gonorrhoeae were inhibited by less than 0.25 microgram/ml. Most enterococci had cefquinome MICs of 4-8 micrograms/ml. Cefquinome was extremely active against group-A streptococci and Streptococcus pneumoniae with MICs less than 0.12 microgram/ml. 90% of methicillin-susceptible Staphylococcus aureus 90% were inhibited by 2 micrograms/ml. Overall, the in vitro activity of cefquinome was comparable with aminothiazolyl cephalosporins. It inhibited some Enterobacter and Citrobacter freundii resistant to ceftazidime as did cefpirome and cefepime. Cefquinome was not destroyed by the common plasmid beta-lactamases TEM-1, TEM-2, SHV-1, or by the chromosomal beta-lactamases of Klebsiella, Branhamella, and Pseudomonas, but it was hydrolyzed by TEM-3, TEM-5, and TEM-9. Its activity was not adversely decreased in different medium or protein, and minimum bactericidal concentrations (MBCs) for most species except for Enterobacter were within a dilution of MICs.     

Antibiotic Susceptibility of Streptococcus bovis and Other Group D Streptococci Causing Endocarditis

Seventy-four strains of Streptococcus bovis and 35 strains of enterococci (Streptococcus faecalis and its varieties, Streptococcus faecium and Streptococcus durans), most of which were isolated from patients with endocarditis, were tested for their susceptibility to penicillin, ampicillin, erythromycin, cephalothin, vancomycin, methicillin, tetracycline, chloramphenicol, kanamycin, streptomycin, and gentamicin. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) were determined by a microtiter broth dilution technique. All of these organisms are group D streptococci, but the S. bovis strains are not enterococci. On the basis of both MIC and MBC, the S. bovis strains were much more susceptibile in general to antibiotics then were the enterococcal strains. For the S. bovis strains, the lowest MICs were obtained with penicillin, ampicillin, and erythromycin, and the lowest MBCs with penicillin and ampicillin. Although these antibiotics were also the most active against the enterococci, the MICs and MBCs were much higher than obtained with the S. bovis strains. Gentamicin was the most active aminoglycoside. On the basis of in vitro susceptibility results, the S. bovis strains resemble the viridans streptococci rather than enterococci.     

MICs and MBCs of clarithromycin against Mycobacterium avium within human macrophages.

The inhibitory and bactericidal activities of clarithromycin were determined quantitatively against the intracellular populations of five Mycobacterium avium strains growing in monocyte-derived human macrophages. The MICs were 1.0 microgram/ml, and the MBCs ranged from 16.0 to 64.0 micrograms/ml; these values were similar to the MICs and MBCs found in broth cultures at pH 7.4 and were substantially lower than those found in broth cultures at pHs 6.8 and 5.0. Since the intracellular environment has a neutral or even an acidic pH, relatively low MICs and MBCs found in macrophage cultures can be associated with the fact that the drug concentrations in macrophages are substantially higher than those in the medium in which these cells are cultivated. Pretreatment of the macrophages 2 days prior to infection decreased the MICs twofold in comparison with results of experiments in which the drug was added to already infected macrophages.     

In vitro activity of LY264826 compared to other glycopeptides and daptomycin.

LY264826 a new naturally occurring glycopeptide inhibited 90% of methicillin-susceptible and -resistant Staphylococcus aureus at 1 micrograms/ml. LY264826 had similar activity against methicillin-susceptible and -resistant coagulase-negative staphylococci. The LY264826 MIC90 for Streptococcus pyogenes was 0.25 microgram/ml, twofold more active than vancomycin and twofold less active than teicoplanin. LY264826 was eightfold more active than vancomycin and twofold more active than teicoplanin against enterococci. LY264826 inhibited Streptococcus pneumoniae at 0.25 microgram/ml and Listeria monocytogenes at 0.5 microgram/ml. Clostridium were inhibited by less than or equal to 0.25 microgram/ml of LY264826 and peptococci, peptostreptococci, and Fusobacterium were inhibited by less than 0.5 microgram/ml. Bacteroides species were LY284826 -resistant as were all Enterobacteriaceae, Flavobacterium, and Neisseria spp. Minimum bactericidal and inhibitory concentrations (MBCs and MICs) were within a dilution for S. aureus, S. pyogenes, and S. pneumoniae, but greater than or equal to 32-fold greater for enterococci.     

Antimicrobial effects of a new carboxyquinolone drug, Q-35, on five serogroups of Leptospira interrogans.

New carboxyquinolone drugs, including the recently developed Q-35, were evaluated for their in vitro potency against five serogroups of Leptospira interrogans. Q-35, ofloxacin, ciprofloxacin, and tosufloxacin showed MICs (0.05 to 0.20 microgram/ml) comparable to those of tetracycline. However, MBCs of these drugs varied between 10- and 100-fold above the MIC for most strains tested. Q-35 was shown to be active against L. interrogans in vitro as judged by the MICs obtained.     

In vitro susceptibility of Mycobacterium kansasii to clarithromycin.

The MICs of the macrolide clarithromycin for 31 clinical isolates of Mycobacterium kansasii were determined by three different methods. The methods employed were the proportion resistance method on 7H10 agar, the radiometric (BACTEC) method, and the T100 method of datum analysis. All methods gave similar results. The MICs were in a narrow range from 0.16 to 0.50 microgram/ml, with the MICs for 90% of isolates tested of 0.50 microgram/ml for the agar dilution and radiometric methods and 0.37 microgram/ml for the T100 method. The MBCs were determined for nine representative isolates by the radiometric broth method. The MBCs were equal to the MICs for four isolates, and the MBCs were twofold higher than the MICs for five isolates. Killing of 99.9% of the bacterial population was achieved at a clarithromycin concentration of 2.0 micrograms/ml for all nine isolates tested.