Klebsiella species: antimicrobial susceptibilities, bactericidal kinetics, and in vitro inactivation of beta-lactam agents.

In vitro properties of 19 antimicrobial agents were tested with 56 isolates of Klebsiella spp. The aminoglycosides and the new beta-lactam compounds cefotaxime and moxalactam were the most inhibitory drugs tested. Chloramphenicol, tetracycline, trimethoprim, and trimethoprim-sulfamethoxazole were moderately active, whereas piperacillin, mezlocillin, and furazlocillin were ineffective against 25% of the isolates. Gentamicin was the only agent tested that was uniformly bactericidal in time-kill experiments with drug concentrations of four times the minimal inhibitory concentration. In combination studies with gentamicin, moxalactam and furazlocillin each increased the rate of bacterial killing for three of five isolates as compared with gentamicin alone, whereas chloramphenicol significantly retarded the rate of bacterial killing for the same number of strains. Furazlocillin was completely inactivated after 24 h of incubation with each of five selected strains. The inactivation of moxalactam, cefoxitin, and cephalothin was 36, 56, and 72%, respectively. In all instances in which these four agents were inactivated to levels below the minimal bactericidal concentration, there was accelerated growth after initial inhibition. However, regrowth also occurred in three instances in which drug levels were higher than the minimal bactericidal concentration. Retesting after drug exposure revealed a 4- to 32-fold rise in the minimal inhibitory concentration and minimal bactericidal concentration in two of these isolates.     

Activity of Netilmicin Compared with Those of Gentamicin and Tobramycin Against Enterobacteria and Pseudomonas aeruginosa

The inhibitory activity of netilmicin against 500 isolates of gram-negative bacteria was compared with those of gentamicin and tobramycin. Netilmicin was considerably less active than tobramycin and slightly less inhibitory than gentamicin for Pseudomonas aeruginosa but was at least as active against Escherichia coli and Klebsiella pneumoniae as were the other two antibiotics. A few Klebsiella and Serratia isolates resistant to gentamicin and tobramycin were inhibited by netilmicin. All three antibiotics were strongly bactericidal for E. coli, K. pneumoniae and P. aeruginosa but had less lethal activity against the otherwise susceptible Serratia isolates tested. Some necessary precautions in reading minimal inhibitory concentrations on agar media are stressed, and some possible advantages of a 4-h bactericidal test, using a constant antibiotic concentration, are defined.     

Serotype distribution of penicillin-resistant pneumococci and their susceptibilities to seven antimicrobial agents

A total of 229 clinical isolates of Streptococcus pneumoniae recovered from 225 patients were serotyped and tested for susceptibility to penicillin G, ampicillin, mezlocillin, cefazolin, erythromycin, clindamycin, chloramphenicol, and sulfamethoxazole-trimethoprim. Of all the isolates, 48 (21.0%) showed intermediate resistance and 17 (7.4%) showed resistance to penicillin G. Penicillin-resistant strains had higher minimal inhibitory concentrations of ampicillin, mezlocillin, and cefazolin than did penicillin-susceptible strains. Resistance to erythromycin and clindamycin was rare (1.3 and 0.9%, respectively). Of the isolates, 8.7% were resistant to sulfamethoxazole-trimethoprim, and all were susceptible to chloramphenicol. Penicillin resistance was associated with 13 serotypes. Serotypes 14, 19F, 19A, and 23F were both highly prevalent and frequently penicillin resistant.     

Interaction of clindamycin and gentamicin in vitro

The minimal inhibitory concentrations of clindamycin and gentamicin alone and in combinations were determined by a microdilution method for 163 aerobic, facultative, and anaerobic clinical isolates. All 77 strains of Staphylococcus aureus, Diplococcus pneumoniae, Streptococcus pyogenes, and anaerobic bacteria (except for three strains of Clostridium) were inhibited by 1.6 mug or less of clindamycin per ml. Gentamicin did not interfere with the activity of clindamycin within the range of concentrations tested (0.1 to 100 mug/ml); for some strains combinations were synergistic. Sixty-two (94%) of 66 strains of Enterobacteriaceae and Pseudomonas aeruginosa were inhibited by 6.2 mug or less of gentamicin per ml. Combinations of clindamycin and gentamicin were indifferent for 29 strains and synergistic for 33 strains. All 20 strains of enterococcus, three strains of Clostridium, three strains of Escherichia coli, and one strain of Proteus rettgeri were resistant to both clindamycin (minimal inhibitory concentration greater than 3.1 mug/ml) and gentamicin (minimal inhibitory concentration greater than 6.2 mug/ml). Combinations of clindamycin and gentamicin were indifferent for 16 and synergistic for 11 of the resistant strains. Except for clindamycin-sensitive isolates, synergy was usually observed only at concentrations of one or both drugs which are not readily obtainable in vivo. Antagonism was never observed.     

In vitro activity of fosfomycin, alone and in combination, against methicillin-resistant Staphylococcus aureus.

We tested 148 strains of clinical isolates of methicillin-resistant Staphylococcus aureus against fosfomycin alone and in combination with methicillin, cefamandole, gentamicin, trimethoprim, and vancomycin. Fosfomycin inhibited 90% of the 148 methicillin-resistant S. aureus strains at a concentration of 4 micrograms/ml. Synergism was observed in 97 strains (66%) with fosfomycin-cefamandole and in 69 strains (46%) with fosfomycin-methicillin. The combinations of fosfomycin with vancomycin, gentamicin, and trimethoprim were indifferent in most strains.     

Susceptibility of Pseudomonas aeruginosa to Tobramycin or Gentamicin Alone and Combined with Carbenicillin

To explore more effective therapy for Pseudomonas aeruginosa, 264 recent clinical isolates were tested by agar dilution using gentamicin and tobramycin alone and combined with carbenicillin to seek synergistic effects. Synergism was defined as a fourfold or greater decrease in the minimal inhibitory concentration of each drug in a pair. At a concentration of 3.12 μg/ml, gentamicin inhibited 73% of the strains and tobramycin inhibited 98%. The gentamicin-carbenicillin combination was synergistically active against 57% of the strains, and tobramycin-carbenicillin was active against 46%. The effect did not correlate with either susceptibility or resistance to gentamicin or tobramycin alone. The data suggest that tobramycin or tobramycin plus carbenicillin may provide alternate therapy where susceptibility to gentamicin or synergism between gentamicin and carbenicillin cannot be demonstrated; however, the degree of susceptibility to either aminoglycoside antibiotic alone cannot be used to predict a synergistic effect.     

Synergism between aminoglycosides and cephalosporins with antipseudomonal activity: interaction index and killing curve method.

Combinations of gentamicin with cefotaxime, moxalactam, and ceftazidime were tested against 43 bacterial strains, most of them blood isolates. With an interaction index of less than or equal to 0.5 as borderline, synergism was demonstrated against 30 to 40% of the strains by the fractional inhibitory concentration index and against 50 to 70% by the fractional bactericidal concentration index. The reproducibility of the index was within +/- 0.2 for two-thirds of 40 repetitive assays and within +/- 0.4 to 0.5 for all of these assays. Similar results were obtained when netilmicin was substituted for gentamicin. The killing curve system for studying antibiotic synergism was standardized to give results comparable to those obtained with the interaction index. This was achieved when one-half of a previously determined minimum bactericidal concentration was used for single drugs and the amount of antibiotic was at least halved again when drugs were used in combination. An initial bacterial concentration of 10(5) to 10(6) colony-forming units per ml is recommended. Given these conditions, synergism could be defined as a 2-log 10 or more decrease in viable count given by both drugs together, as compared with the more active of the pair after 24 h. Prediction of killing curve results could then be obtained with the fractional bactericidal concentration index. When cephalosporins and gentamicin were combined from the start, the beta-lactam antibiotics were less susceptible to inactivation, as demonstrated in time-killing assays. If one of the antibiotics were added after 24 h, synergism was not demonstrable. The results indicate that the new cephalosporins may be advantageously combined with aminoglycosides.     

Penicillin tolerance of human isolates of group C streptococci.

Seventeen clinical isolates of group C streptococci were tested for penicillin tolerance. Sixteen of the strains showed penicillin tolerance with a 32-fold or greater difference between the minimal inhibitory concentration and the minimal bactericidal concentration. Synergism was demonstrated with a combination of penicillin and gentamicin for all 17 strains tested. The rate of antibiotic killing was measured for five of the streptococcal strains by using the combination of penicillin and gentamicin. All isolates were killed within 5 h with the combination, but viable organisms were recovered after 48 h when either drug was used alone. Our study suggests that penicillin tolerance with group C streptococci may occur frequently and may account for the poor outcome of serious group C streptococcal infections tested with penicillin alone.     

Minimum Bactericidal Concentration of Sulfamethoxazole-Trimethoprim for Haemophilus influenzae: Correlation with Prophylaxis

The inability of sulfamethoxazole-trimethoprim (SXT) to eradicate Haemophilus influenzae nasopharyngeal carriage in all asymptomatic patients in closed populations was examined in vitro. A broth medium was adapted for susceptibility testing of H. influenzae which permitted us to determine minimum inhibitory concentrations and minimum bactericidal concentrations (MBCs). The minimum inhibitory concentrations were all low, but the MBCs were bimodally distributed. Trimethoprim alone or the combination SXT either was bactericidal for H. influenzae isolates at low concentrations (i.e., low MBCs) similar to minimum inhibitory concentrations or showed no bactericidal activity (i.e., high MBCs). If trimethoprim was bactericidal when tested alone against H. influenzae, then the combination SXT was also bactericidal. H. influenzae carriage could not be eradicated from asymptomatic patients with SXT therapy when that combination was not bactericidal for these isolates in vitro. H. influenzae carriage was eradicated from patients when the activity of SXT was bactericidal in vitro. H. influenzae strains that are not killed by trimethoprim or SXT seem to occur at random.     

In Vitro Evaluation of Tobramycin, a New Aminoglycoside Antibiotic

One hundred fifty-two strains of Escherichia coli, Klebsiella-Enterobacter, Pseudomonas aeruginosa, Proteus species, and Staphylococcus aureus were inhibited by 3.1 mug of tobramycin/ml in a broth-dilution method and showed zones of inhibition of 16 mm or more around a 10-mug tobramycin disc in the Kirby-Bauer method. Tobramycin was most active against S. aureus, 100% of strains being inhibited by 0.1 mug/ml. All strains of E. coli, K. pneumoniae, P. aeruginosa, and indole-positive Proteus species, and 80% of Enterobacter species were inhibited by 0.8 mug of tobramycin/ml, whereas only 48% of P. mirabilis strains were inhibited by this concentration. Tobramycin was approximately twice as active as gentamicin against S. aureus, four times as active against P. aeruginosa, slightly more active against E. coli and Enterobacter species, equally active against P. mirabilis, and slightly less active against K. pneumoniae. The minimal bactericidal concentrations of tobramycin and gentamicin were the same as or twice the minimal inhibitory concentrations for all strains except those of P. aeruginosa, against which greater concentrations of both gentamicin and tobramycin were required for bactericidal activity. Tobramycin sterilized cultures of S. aureus, E. coli, and P. aeruginosa, but the rate of bactericidal action was faster with a combination of tobramycin and carbenicillin than with either antibiotic alone in the same concentrations. Tobramycin retained potency in the presence of 200 to 600 mug of carbenicillin/ml for at least 6 hr of incubation at 37 C, but lost potency in the presence of 600 mug of carbenicillin/ml by 24 hr of incubation and in the presence of 800 mug/ml by 2 hr of incubation.     

Comparative Synergistic Activity of Nafcillin, Oxacillin, and Methicillin in Combination with Gentamicin Against Enterococci

The effectiveness of three semisynthetic, penicillinase-resistant penicillins alone and in combination with gentamicin was tested against 29 clinical isolates of enterococci. The minimal inhibitory concentrations of nafcillin were considerably lower than those of oxacillin and methicillin but were slightly higher than those of penicillin. At clinically achievable concentrations, the combination of nafcillin plus gentamicin produced enhanced killing against 13 of 14 strains of enterococci and was synergistic (by very rigid criteria) against 10 of 14 strains. In contrast, combinations of oxacillin plus gentamicin were synergistic against only 3 of 14 strains, and methicillin plus gentamicin produced synergistic killing against only 1 of 14 strains.     

Synergistic interaction of trimethoprim and sulfamethoxazole on Paracoccidioides brasiliensis.

The in vitro interaction of trimethoprim and sulfamethoxazole on clinical isolates of Paracoccidioides brasiliensis was studied. With complete inhibition and a visual endpoint used as the criteria, three of four strains had minimal inhibitory concentrations that indicated resistance to sulfamethoxazole, and all four strains were resistant to trimethoprim. A marked synergism in inhibition was noted with the combination of these drugs against sulfa-resistant strains. A sulfamethoxazole/trimethoprim ratio of 5:1 was the most synergistic. Fifty percent inhibition, determined spectrophotometrically, of the strains could be achieved with sulfamethoxazole alone. In summary, the striking synergy observed suggests that combination chemotherapy with these drugs deserves further study.     

Antibiotic synergism against Listeria monocytogenes

The effectiveness of ampicillin, penicillin, streptomycin, and gentamicin against 20 strains of Listeria monocytogenes was studied in vitro. For all strains, the minimal bactericidal concentration (MBC) of both ampicillin and penicillin was much higher than the minimal inhibitory concentration (MIC). The MBC of both streptomycin and gentamicin was close to the MIC, but relatively high concentrations of these antibiotics were necessary to inhibit the growth of most of the strains of Listeria. The combination of penicillin plus streptomycin was synergistic against 19 of 20 strains and in the remaining strain produced enhanced killing (but of less magnitude than our criterion for synergism). Combinations of penicillin plus gentamicin, ampicillin plus streptomycin, and ampicillin plus gentamicin produced enhanced killing against all strains tested. No antagonism was observed when ampicillin or penicillin was combined with streptomycin or gentamicin.     

LY-127935: a novel beta-lactam antibiotic with unusual antibacterial activity.

The in vitro activity of LY-127935, a new beta-lactam antibiotic, was examined by using 370 clinical bacterial strains. In comparison with several other beta-lactam agents, LY-127935 was the most inhibitory against the Enterobacteriaceae. It was remarkably active against multi-drug-resistant strains of Enterobacter spp., Serratia spp., and Pseudomonas aeruginosa. LY-127935 had four- to eightfold greater activity than did cefoxitin against Bacteroides fragilis. Production of beta-lactamase by Enterobacteriaceae did not influence the minimal inhibitory concentration of LY-127935. However, the beta-lactamase-producing strains of B. fragilis and Haemophilus influenzae had generally higher minimal inhibitory concentrations. LY-127935 was the least active agent tested against gram-positive aerobic cocci. Variations in pH, salt content, protein content, or inocula size had little influence on susceptibility to LY-127935. Although combination studies with LY-127935 and gentamicin demonstrated synergy for P. aeruginosa, the rates of killing for the combination and for gentamicin alone were similar.     

Bacteriostatic and bactericidal activities of beta-lactam antibiotics enhanced by the addition of low concentrations of gentamicin

The combined effects of low concentrations of gentamicin on certain beta-lactam antibiotics were studied by the agar plate method. the combination of gentamicin with cephalothin produced synergism against 17 of 26 strains of Escherichia coli and 19 of 27 strains of Klebsiella sp. if assessed at the bacteriostatic (minimal inhibitory concentration) level. Synergy against many more strains was apparent when bactericidal concentrations were used. Synergy against most of these strains was observed if bactericidal concentrations with brief exposure times (3 h) to the antibiotics were used for measurement. Additive effects were observed in almost all of the remaining strains. The combination of gentamicin and carbenicillin were synergistic against most strains of Pseudomonas aeruginosa when any bacteriostatic or bactericidal measurement was used as the criterion.     

Susceptibility of Clostridium botulinum to thirteen antimicrobial agents.

A total of 224 strains of Clostridium botulinum (including isolates from 14 patients with infant botulism and 4 with wound botulism) and 15 strains of C. sporogenes were tested by agar dilution for susceptibility to tetracycline, metronidazole, erythromycin, penicillin, rifampin, chloramphenicol, clindamycin, cephalothin, cefoxitin, vancomycin, sulfamethoxazole-trimethoprim, nalidixic acid, and gentamicin. At least 90% of the C. botulinum strains tested (except for nonproteolytic strains of toxin type F with penicillin) were susceptible to all drugs except sulfamethoxazole-trimethoprim, nalidixic acid, and gentamicin. Minimal inhibitory concentrations for strains from patients with infant and wound botulism were similar to those for other C. botulinum strains.     

In Vitro Activity of LY127935

LY127935 is a unique new beta-lactam antibiotic. Its activity against 536 clinical isolates was studied by using microdilution methods of susceptibility testing and compared with the activities of cefamandole, cefoxitin, and cephalothin. The lowest concentrations required to inhibit at least 90% of strains tested (MIC(90)s) of LY127935 for Staphylococcus aureus, Streptococcus pyogenes, Streptococcus agalactiae, and Streptococcus pneumoniae ranged from 2 to 8 mug/ml. The MIC(90)s for other staphylococci and streptococci were higher. The MIC(90)s for Enterobacteriaceae and Pseudomonas species ranged from 0.12 to 8 mug/ml and 8 to >32 mug/ml, respectively. The MIC(90)s for anaerobes ranged from 2 to >32 mug/ml. As determined by MIC(90)s, LY127935 was consistently the least active antibiotic against facultatively anaerobic gram-positive cocci and the most active against aerobic and facultatively anaerobic gram-negative bacilli. Its position with respect to activity against anaerobes varied from being the most active against Bacteroides fragilis and Clostridium perfringens to the least active against anaerobic cocci. In a population of multidrug-resistant isolates, concentrations of 8 mug or less of LY127935 per ml inhibited 82% of Enterobacteriaceae; concentrations of 32 mug or less per ml inhibited 100% of Enterobacteriaceae and 40% of P. aeruginosa. Increasing the inoculum size by 100-fold did not increase the minimal inhibitory concentrations of LY127935 or cefoxitin but did increase minimal inhibitory concentrations of cefamandole and cephalothin for some Enterobacteriaceae. All four drugs were bactericidal; minimal bactericidal concentrations were the same or one concentration higher than minimal inhibitory concentrations for 91 to 96% of strains tested. The broad spectrum and marked in vitro activity of LY127935 make it a promising new antibiotic.     

2006年中国七家教学医院革兰阳性球菌耐药性研究

目的 调查2006年我国革兰阳性球菌的耐药性.方法 收集2006年6-12月全国7家教学医院分离的非重复革兰阳件菌674株,以琼脂稀释法测定抗菌药物的MIC.结果 100株肺炎链球菌中,青霉素耐药的肺炎链球菌(PRSP)和青霉素中介的肺炎链球菌(PISP)分别占1%和19%.所有肺炎链球菌均对万古霉素和替考拉宁高度敏感;对左氧氟沙星和莫西沙星的敏感率分别为97%和98%;对阿莫西林/克拉维酸、头孢曲松和氯霉素的敏感率依次为96%、87%和73%;青霉素敏感的肺炎链球菌(PSSP)全部对头孢丙烯和头孢克罗的敏感率分别为62.0%和55.7%,而PISP和PRSP对这2种抗菌药物均耐药;所有肺炎链球菌埘大环内酯类、甲氧苄啶/磺胺甲恶(噁)唑(TMPCo)和四环素的敏感率均小于35%.金黄色葡萄球菌和凝固酶阴性葡萄球菌(SCON)中耐甲氧西林菌株分别占48%(33%～84%)和81%(69%～94%).耐甲氧西林金黄色葡萄球菌(MRSA)对TMPCo、氯霉素和利福平敏感率分别为72%、66%和45%;对大环内脂类、氨基糖苷类、四环素和氟喹诺酮类的敏感率低于18%.粪肠球菌和屎肠球菌对高浓度庆大霉素耐药率分别为56%(30%～86%)和80%(50%～100%).未发现对万古霉素和替考拉宁耐药的葡萄球菌和粪肠球菌;但在杭州发现对该2种抗菌药物同时耐药的屎肠球菌.除氯霉素和四环素外,粪肠球菌对其他所测抗菌药物的敏感率均高于屎肠球菌.结论 各地区革兰阳件耐药性有所差异,且对大部分抗菌药物的耐药率较2005年本项目的 监测结果有所升高.替考拉宁和万古霉素对革兰阳性球菌仍保持很高的抗菌活性.     

Comparative Activity of Tobramycin, Amikacin, and Gentamicin Alone and with Carbenicillin Against Pseudomonas aeruginosa

The effect of gentamicin against 130 clinical isolates of Pseudomonas aeruginosa was compared with that of two investigational aminoglycoside antibiotics, tobramycin and amikacin. Minimal inhibitory concentration data indicated that, on a weight basis, tobramycin was two to four times as active as gentamicin against most isolates. However, 14 of 18 organisms highly resistant to gentamicin (>/=80 mug/ml) were also highly resistant to tobramycin. Amikacin was the least active aminoglycoside on a weight basis, but none of the isolates were highly resistant to this antibiotic. When therapeutically achievable concentrations were used, adding carbenicillin to gentamicin or to tobramycin enhanced inhibitory activity against those isolates susceptible (相似文献   

Resistance to Six Aminoglycosidic Aminocyclitol Antibiotics Among Enterococci: Prevalence, Evolution, and Relationship to Synergism with Penicillin

Two hundred and three recent clinical isolates of enterococci were tested for susceptibility to streptomycin, kanamycin, amikacin, gentamicin, sisomicin, and tobramycin. Depending upon the source of the isolate, 36 to 54% of the enterococci demonstrated high-level resistance (minimal inhibitory concentration, >2,000 mug/ml) to streptomycin, 16 to 49% to kanamycin, and 0 to 14% to amikacin. None of the strains was highly resistant to gentamicin, sisomicin, or tobramycin. A comparison with isolates of enterococci obtained in 1968 revealed that there has been a decrease in prevalence of high-level resistance among organisms isolated from wound cultures in 1976. However, no decrease in resistance to streptomycin or kanamycin was demonstrated among blood or urine isolates. Penicillin, combined with gentamicin, sisomicin, or tobramycin, was synergistic against all 10 strains of Streptococcus faecalis subjected to formal testing. For streptomycin and kanamycin, the presence or absence of synergism with penicillin correlated with the absence or presence of high-level aminoglycoside resistance. High-level resistance to amikacin was seen in only 1 of the 10 strains. Nonetheless, combinations of penicillin plus amikacin failed to produce synergistic killing against 6 of the 10 strains. Indeed, the combination was synergistic only against those four strains that were susceptible to high levels of kanamycin.