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

Abstract

Cefaclor appeared to be an effective antibiotic against both beta-lactamasepositive and beta-lactamase-negative strains of Haemophilus influenzae type b when tested with 105 colony-forming units per ml. With inocula in excess of 106 colony-forming units per ml, these organisms were neither inhibited nor killed at concentrations of 400 ,ug/ml. This inoculum effect was also demonstrated in timekill curve studies.Cefaclor, an oral cephalosporin derivative with antibacterial activity against Haemophilus influenzae, has become a popular therapeutic choice for otitis media and for systemic H. influenzae infection after initial parenteral treatment with other agents in children. This study demonstrates the in vitro activity of cefaclor against H. influenzae type b, including betalactamase-producing strains, the effect of inoculum size, and the growth kinetics of these organisms in the presence of cefaclor.Thirty strains of H. influenzae type b, including fifteen beta-lactamase-positive strains, were tested for susceptibility to cefaclor. Twenty-nine of these isolates were obtained from blood or cerebrospinal fluid samples from children with infection, and one was cultured from an infected eye sample. The definitions of minimal inhibitory and bactericidal concentrations and the methodology used have been described previously (1). Minimal inhibitory and bactericidal concentrations were determined for three inoculum sizes, 104, 106, and 107 colony-forming units (CFU)/ml, prepared by diluting an overnightgrowth suspension of the organisms adjusted to a McFarland no. 0.5 turbidity standard.Time-kill curves were determined with cefaclor for five beta-lactamase-positive and five beta-lactamase-negative H. influenzae type b strains with both light (104 to i05 CFU/ml) and heavy (106 to 107 CFU/ml) inocula. The organisms were grown overnight in Mueller-Hinton broth with 4% Fildes enrichment. A concentration of 2 ,ug/ml was chosen for the cefaclor because it correlated with achievable middle-ear fluid levels obtained after oral therapy (5). Each test sample (total volume, 4 ml) was incubated at 37°C in a candle jar, and 0.5-ml portions were removed at 0, 4, 8, and 24 h. These were serially diluted and streaked onto Fildes-enriched Mueller-Hinton agar. Colonies were counted after 24 h of incubation. Specimens from antibiotic-free control tubes were sampled simultaneously, and colony counts were determined in the same manner.With the standard inoculum (104 to 105 CFU/ml), 100%o of both beta-lactamase-positive and beta-lactamase-negative strains were inhibited by 1.56 pg of cefaclor per ml, and 97% were killed with 3.12

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

“…This effect seemed more pronounced with 3-lactamase-producing strains (11,13,15). Levin et al showed in vitro resistance of H. influenzae type b to cefaclor with inocula in excess of 106 CFU/ml, regardless of 1-lactamase production (9). The inoculum effect may also vary, depending on the testing method.…”

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

“…Its activity is at least 16 times more effective than that of cefaclor, the other oral cephalosporin compared. Unlike the MICs of cefaclor, which were reported to be affected by an increase in inoculum size (1,5), the MICs of Ro 15-8074 were unaffected when the inoculum of gonococcal strains was raised to 106 CFU. In view of the good in vitro activity of Ro 15-8074 against both PPNG and non-PPNG strains, its stability to gonococcal 3-lactamase, and the fact that peak levels of Ro 15-8074 in serum after oral administration of 1 g of Ro 15-8075 is 5 mg/liter (data on file, Hoffmann-La Roche Inc.), which is approximately 50 times the concentration required to inhibit all the N. gonorrhoeae strains tested in our laboratory, this new oral cephalosporin warrants further investigation.…”

Bactericidal activities of chloramphenicol and eleven other antibiotics against Salmonella spp

“…In addition, the bacteriostatic activity of chloramphenicol should be considered when treating other serious Salmonella infections, especially meningitis (5,15,18,19 (23,26) as well as after changes in broth (17), temperature (9), pH (25), and inoculum density (11,16). Although the changes in MBCs noted after prolonged incubation may simply be in vitro phenomena, it is intriguing to speculate that similar changes do occur in the blood, cerebrospinal fluid, and tissues, assuming that it may take various times for susceptible organisms to be killed once they are exposed to a given antimicrobial agent.…”

“…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 MBCs 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.…”