Susceptibility of Haemophilus influenzae type b to cefaclor and influence of inoculum size

Inoculum effect of β-lactam antibiotics

The phenomenon of attenuated antibacterial activity at inocula above those utilized for susceptibility testing is referred to as the inoculum effect. Although the inoculum effect has been reported for several decades, it is currently debatable whether the inoculum effect is clinically significant. The aim of the present review was to consolidate currently available evidence to summarize which β-lactam drug classes demonstrate an inoculum effect against specific bacterial pathogens. Review of the literature showed that the majority of studies that evaluated the inoculum effect of β-lactams were in vitro investigations of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Across all five pathogens, cephalosporins consistently displayed observable inoculum effects in vitro, whereas carbapenems were less susceptible to an inoculum effect. A handful of animal studies were available that validated that the in vitro inoculum effect translates into attenuated pharmacodynamics of β-lactams in vivo. Only a few clinical investigations were available and suggested that an in vitro inoculum effect of cefazolin against MSSA may correspond to an increased likeliness of adverse clinical outcomes in patients receiving cefazolin for bacteraemia. The presence of β-lactamase enzymes was the primary mechanism responsible for an inoculum effect, but the observation of an inoculum effect in multiple pathogens lacking β-lactamase enzymes indicates that there are likely multiple mechanisms that may result in an inoculum effect. Further clinical studies are needed to better define whether interventions made in the clinic in response to organisms displaying an in vitro inoculum effect will optimize clinical outcomes.

Characterization of the inoculum effect with Haemophilus influenzae and beta-lactams

An inoculum effect is defined as a four-fold or greater increase in MIC with an increase in bacterial inocula. Haemophilus influenzae was tested for an inoculum effect with ampicillin, cefuroxime, and amoxicillin/clavulanate using the standard initial inocula (5 x 10(5) CFU/mL) and a higher initial inocula (1 x 10(7) CFU/mL). An inoculum effect was observed with both beta-lactamase (TEM-1, ROB-1) positive and beta-lactamase negative strains of H. influenzae when MICs were determined based on turbidity. MICs based on viable cell counts however, demonstrated that only beta-lactamase positive strains of H. influenzae produced an inoculum effect. These observations suggest that MICs determined based on turbidity, using high initial inocula, are not reliable when examining the inoculum effect in H. influenzae. The magnitude of the inoculum effect with beta-lactamase positive strains was beta-lactam dependent (ampicillin > amoxicillin/clavulanate > cefuroxime). beta-lactam kill-curves confirmed the aforementioned results. Addition of the beta-lactamase inhibitor clavulanate completely reversed the inoculum effect in beta-lactamase (TEM-1 and ROB-1) positive strains of H. influenzae with all beta-lactams tested. Introduction of the beta-lactamase gene TEM-1 on plasmid vector pLS88 into a beta-lactamase negative strain of H. influenzae (Rd) produced an inoculum effect based on viable cell counts. In conclusion, our results suggest that the beta-lactam inoculum effect demonstrated by H. influenzae is the result of beta-lactamase production and is poorly assessed by turbidity.

Antibiotic resistance in Haemophilus influenzae: mechanisms, clinical importance and consequences for therapy

Invasive and non-invasive infections caused by Haemophilus influenzae are frequently diagnosed in children below the age of 5 years. The treatment of choice for these infections was ampicillin. However, since the early 1970s the increasing prevalence of resistance to ampicillin and other antibiotics has necessitated major changes in antibiotic therapy. This article summarizes some of the important clinical features of diseases caused by H. influenzae. The epidemiology, the problems with in vitro susceptibility testing and the mechanisms of resistance to major antibiotics are reviewed. The consequences of antibiotic resistance for the treatment of diseases caused by H. influenzae are discussed.

Surveillance study of resistance in Haemophilus species in Greece

The results are presented of the first surveillance study in Greece on resistance in strains of Haemophilus influenzae (n = 61) and Haemophilus parainfluenzae (n = 96) to six antibiotics. Strains were isolated in a three-month period in 1987 in most cases from sputum specimens from adult patients with lung infections. The overall rate of resistance to ampicillin was 28.3%, to chloramphenicol 2.7%, to cefaclor 2.7%, to tetracycline 10.6%, to erythromycin 38.1% and to cotrimoxazole 5.3%. It is evident that the resistance rate of Haemophilus spp. is steadily increasing in Greece as in other European countries. Regular surveys of resistance patterns for both older and newer antimicrobial agents are therefore necessary.

In vitro activity of Ro 15-8074 and Ro 19-5247 in comparison to cefaclor and cefalexin

805 clinical isolates were investigated for their in vitro sensitivity against Ro 15-8074 and Ro 19-5247 in comparison to cefaclor and cefalexin in a serial dilution test on solid medium. Ro 19-5247 had the strongest activity of all drugs tested against streptococci (except Streptococcus faecalis) and was as active as cefaclor and cefalexin against most strains of Staphylococcus aureus. Ro 19-5247 was the only oral cephalosporin active against Bordetella pertussis. It was on average 160 times more active than cefaclor against Haemophilus influenzae. In its activity against enterobacteria Ro 19-5247 was always superior to cefaclor and cefalexin. Only a few strains of Enterobacter aerogenes, Enterobacter cloacae, Klebsiella pneumoniae, Proteus vulgaris and Serratia marcescens were resistant to Ro 19-5247 as were all strains of Enterobacter agglomerans and Klebsiella ozaenae. Ro 15-8074 was inactive against staphylococci but ten times more active than cefaclor and cefalexin against Streptococcus pyogenes. There was no difference in the activity against Streptococcus pneumoniae and Streptococcus agalactiae. Against Haemophilus influenzae Ro 15-8074 acted 12 times stronger than cefaclor and 100 times stronger than cefalexin. The activity against enterobacteria corresponded to that of Ro 19-5247. Ro 15-8074 was also active against most strains of Enterobacter cloacae and Proteus vulgaris which were resistant to cefaclor and cefalexin.

In vitro activity of Ro 15-8074, a new oral cephalosporin, against Neisseria gonorrhoeae

Ro 15-8074, a new cephalosporin the pivaloyloxymethylester of which (Ro 15-8075) is orally absorbable, showed greater in vitro activity than cefaclor against 48 Neisseria gonorrhoeae strains, including 25 penicillinase-producing strains. Unlike cefaclor, Ro 15-8074 was unaffected by increase in inoculum size, and it exhibited a remarkable stability against gonococcal beta-lactamase hydrolysis.

Bactericidal activities of chloramphenicol and eleven other antibiotics against Salmonella spp

The bactericidal activity of chloramphenicol against 27 strains of Salmonella typhi and 33 strains of S. enteritidis was compared with those of 11 other antibiotics. The geometric mean bactericidal concentrations of chloramphenicol against susceptible strains (36.10 and 43.13 micrograms/ml for S. typhi and S. enteritidis, respectively) far exceeded those of the other 11 antibiotics, with cephalothin having the next highest values (2.67 and 8.66 micrograms/ml) and moxalactam (0.09 and 0.28 micrograms/ml), cefotaxime (0.08 and 0.28 micrograms/ml), ceftriaxone (0.07 and 0.16 micrograms/ml), norfloxacin (0.06 and 0.10 micrograms/ml), and aztreonam (0.05 and 0.20 micrograms/ml) having the lowest values. The results for imipenem (0.24 and 0.81 micrograms/ml) and ceftazidime (0.22 and 0.75 micrograms/ml) were lower than those noted for trimethoprim-sulfamethoxazole (1.20 and 5.56 micrograms/ml), cefamandole (0.62 and 3.29 micrograms/ml), and ampicillin (0.55 and 2.78 micrograms/ml). The MBC of chloramphenicol for some isolates decreased with increased incubation times such that the proportion of susceptible isolates killed by chloramphenicol at concentrations within achievable levels in blood increased from 10% after 24 h to 26% after 48 h of incubation. Although the MBC of the other 11 antibiotics for some isolates were also lowered by prolonged incubation, all 24-h values were within achievable levels in blood. The data indicate that chloramphenicol is not uniformly bacteriostatic against S. typhi and S. enteritidis. The in vivo significance of demonstrating delayed killing by chloramphenicol is, however, uncertain.

Effect of inoculum size on Haemophilus influenzae type b susceptibility to new and conventional antibiotics

Thirty-three Haemophilus influenzae type b isolates, including beta-lactamase acetyltransferase-positive strains, were tested by microtiter broth dilution for susceptibility to eight beta-lactam compounds and chloramphenicol. All antibiotics except ampicillin and chloramphenicol were highly bactericidal against all isolates at an inoculum of 10(5) CFU/ml. However, at an inoculum of 10(5) CFU/ml, the minimal bactericidal concentrations of all drugs except ceftriaxone were above levels usually achievable in cerebrospinal fluid. Results of time-kill studies confirmed this inoculum effect. In vivo studies are needed to test the clinical impact of these observations.