Effect of inoculum size and beta-lactamase production on in vitro activity of new cephalosporins against Haemophilus species

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.

Effects of various test media on the activities of 21 antimicrobial agents against Haemophilus influenzae

As considerable variation in the antimicrobial susceptibility of Haemophilus influenzae has been reported, the effects of various test media on the susceptibility of H. influenzae were studied. MICs were determined by three laboratories for 21 antimicrobial agents against a panel of 100 selected isolates. Testing was performed using a reference NCCLS frozen broth microdilution method with Haemophilus test medium (HTM) broth and dried commercial MIC trays rehydrated with the following media: in-house and commercially prepared HTM broth, Mueller-Hinton broth with 2% lysed horse blood and NAD, IsoSensitest broth with 2% lysed horse blood and NAD, and IsoSensitest broth-based HTM. Overall, all results were very reproducible, with the MIC at which 50% of the isolates tested are inhibited (MIC(50)), MIC(90), and geometric mean MIC being within one doubling dilution by all six methods and at all three testing centers for 15 of the 21 agents tested. Interlaboratory differences were more marked than intralaboratory differences or differences among media. Cefprozil, cefaclor, and trimethoprim-sulfamethoxazole results differed the most, while results for ampicillin, amoxicillin-clavulanic acid, cefdinir, cefixime, ceftriaxone, and clarithromycin were the most reproducible. However, these variations in results caused considerable differences in susceptibility rates for agents for which NCCLS susceptible breakpoints were close to the geometric mean MIC, particularly for cefaclor and cefprozil. This was much less of a problem when pharmacokinetic-pharmacodynamic breakpoints were used. Reproducible susceptibility results were obtained for a wide range of agents against H. influenzae in three laboratories using a variety of media that support the growth of this fastidious species.

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.

Parameterization of inoculum effect via mathematical modeling: aminoglycosides against Staphylococcus aureus and Escherichia coli

Inoculum effect describes the inoculum size dependent changes in minimum inhibitory concentrations (MIC) exhibited by antibiotic-bacterium combinations demonstrating such effect. Traditionally, inoculum effect has been loosely defined based on the extent of increase in the MIC with respect to the increase in inoculum size. In most studies, assessment of MIC data has relied on the arbitrary selection of a point of reference for both baseline MIC and inoculum size. More importantly, this conventional method of assessment does not permit information conveyed in a complete MIC versus inoculum size profile to be fully explored. To undertake these issues, a mathematical model was developed for the description of the entire inoculum effect profile. With the employment of three key parameter estimates, i.e., the baseline MIC, the threshold inoculum size at which the increase in MIC commences, and the rate of increase in MIC with respect to inoculum size, both the shape and location of the profile could be adequately defined. To verify the application of this model, a series of four aminoglycosides were tested against standard strains of E. coli and S. aureus. Results showed a good degree of organism specificity and antibiotic-class dependency of the inoculum effect profiles. Analysis of the parameter estimates obtained provided further support for these observations. In conclusion, the mathematical model developed in the present study adequately described the inoculum effect exhibited by the various aminoglycoside-bacterium combinations tested. The parameter estimates generated by the modeling approach allowed comparison and quantitative analysis of the inoculum effect profiles with minimal difficulties.

Evaluation of Haemophilus influenzae isolates with elevated MICs to amoxicillin/clavulanic acid

A 1994 to 1995 national Haemophilus influenzae surveillance study of 1910 strains showed that 13 strains (0.7%) were resistant to amoxicillin/clavulanic acid (MIC, > or = 8/4 micrograms/ml). These and other selected strains were investigated further in this study. Susceptibility of the surveillance study strains was determined with the commercial microdilution trays used in the original study and in triplicate with reference broth microdilution trays prepared by the investigators, as well as by Etest and disk diffusion. Amoxicillin/clavulanic acid resistance was confirmed for only one of the surveillance study strains. This strain produced double zones of growth with Etest and disk-diffusion methods, with the double zone containing spheroplasts. When the amoxicillin/clavulanic acid MICs of all beta-lactamase positive strains were compared, MIC results obtained with surveillance study trays and the Etest were one to two dilutions higher than MICs obtained with reference trays. The distribution and modal amoxicillin/clavulanic acid MICs of beta-lactamase-positive and -negative strains was essentially the same for a comparison group of strains using reference trays, in contrast to a fourfold higher modal MIC for beta-lactamase-positive strains using surveillance study reagents and strains. These differences in MICs could be attributed to variations in inoculum, the presence of spheroplasts, and/or a difference in potency of amoxicillin and/or clavulanic acid in the tray and Etest reagents used. Methods for assessing the adequacy of the clavulanic acid content are not adequate, amoxicillin control values and a beta-lactamase-positive H. influenzae control strain are required.

Optimal times above MICs of ceftibuten and cefaclor in experimental intra-abdominal infections

The duration of time that serum drug levels remain above the MIC (time above the MIC) for the pathogen has been shown to be the most significant parameter determining the efficacies of beta-lactam antibiotics. In the described study, we investigated the optimal time above the MIC of ceftibuten and cefaclor using a nonneutropenic mouse model of intra-abdominal infections caused by Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Streptococcus pneumoniae. The abilities of the drugs to protect mice against the organisms were determined in mouse protection tests, and the doses were fractionated to produce various dosing regimens with different times above the MIC. All drug-organism combinations showed a significant correlation (r > 0.9) between drug efficacy and the time above the MIC. Also, with ceftibuten treatment, the different dosing regimens that produced equal times above the MIC resulted in the same efficacy, whereas with cefaclor, an apparent dose-dependent effect was observed. These results showed that for a 100% recovery from K. pneumoniae and E. coli infections, the optimal times above the MIC with ceftibuten treatment were 2.2 and 1.6 h, respectively. Relatively high doses of both antibiotics were required to ensure recovery from S. pneumoniae infections. In vitro time-kill studies demonstrated that cefaclor exhibits a marked inoculum effect against the pathogens, and there was a concentration-dependent killing at a large inoculum size. On the other hand, ceftibuten showed no inoculum effect. It is suggested that optimization of both dose and time above the MIC appears to be necessary for the treatment of S. aureus infections with cefaclor, and this may apply to other beta-lactams tht exhibit marked inoculum effects.

Comparison of the inoculum effect of cefoxitin and other cephalosporins and of beta-lactamase inhibitors and their penicillin-derived components on the Bacteroides fragilis group

We compared the inoculum effects for 109 recent clinical isolates of the Bacteroides fragilis group of cefoxitin, cefotetan, ceftizoxime, ceftriaxone, and three beta-lactamase inhibitors (clavulanic acid, sulbactam, and tazobactam) and their penicillin-derived components. Bactericidal activity was assayed and morphologic changes were monitored for selected strains exhibiting a large inoculum effect. Ceftizoxime demonstrated the largest inoculum effect, followed by cefotetan and ceftriaxone. The large inoculum effect of ceftizoxime and ceftriaxone was correlated with filamentous transformation at the high inoculum (10(8) CFU/ml) and lack of bactericidal activity suggesting drug destruction or inactivation. Cefotetan was bactericidal for B. fragilis isolates but not for other members of the B. fragilis group. Cefoxitin showed the least inoculum effect and was consistently bactericidal at high (10(8) CFU/ml), standard (10(6) CFU/ml), and low (10(4) CFU/ml) inocula, followed by ampicillin-sulbactam. Piperacillin-tazobactam and ticarcillin-clavulanic acid showed an intermediate inoculum effect. The degree of inoculum effect observed generally correlated with bactericidal activity at all inocula.

Ampicillin killing curve patterns for ampicillin-susceptible nontypeable Haemophilus influenzae strains by the agar dilution plate count method

Ampicillin killing curve patterns for 20 strains of ampicillin-susceptible nontypeable Haemophilus influenzae were determined by the agar dilution plate count method. The paradoxical effect was detected in the 24-h killing curve patterns for each strain. For the biphasic effect, minimum survivor percentages (maximum killing) occurred over a narrow range of ampicillin concentrations immediately above the MIC, with survivor percentages then rising rapidly to peak at approximately 1-log10-unit increment higher. The 24-h minimum survivor percentages for the 20 strains ranged from approximately 0.01% (rapid killing) to greater than 10% (slow killing). In comparison with the previous results for typeable strains, the present findings suggest that nontypeable stains are, on average, killed much more slowly. Based on the initial 24-h killing curve patterns for the 20 strains, 4 strains were selected as putative representatives of the range of bactericidal responses encountered. These strains were then studied to examine the reproducibility of the 24-h patterns and to determine sequential killing curves. These patterns were found to be reproducible and served to characterize the relative killing responses of the strains. In the sequential studies of three of the four strains, tiny colonies having the gross and microscopic characteristics of L-forms were found to be present on the agar dilution plate count plates prior to the application of penicillinase at 48 and 72 h. Such colonies reverted to vegetative forms within 24 to 48 h after application of penicillinase to the panels. Of particular interest was the observation that the paradoxical effect was manifested both by the L-form colonies and by the reverted vegetative colonies. The late development of L-forms was observed for both rapidly and slowly killed strains.

Haemophilus influenzae: antibiotic susceptibility

Ampicillin resistance was first reported among clinical isolates of Haemophilus influenzae in 1972. Reports of chloramphenicol resistance followed shortly thereafter. The principal mechanism of resistance to these two antibiotics is enzymatic. Although other mechanisms have been described, they are found in comparatively few strains. The genetic information for the inactivating enzymes is plasmid mediated and therefore readily transmissible to susceptible strains. Consequently, effective therapy for invasive disease caused by this pathogen has been seriously compromised. As antibiotic susceptibility became less predictable, in vitro testing became increasingly important. Unfortunately, the standardization of methods for laboratory testing has been slow and complicated by the fastidious nature of the organisms. This review traces the development of antibiotic resistance in H. influenzae, discusses the mechanisms which appear to be important in mediating resistance, explores newer antimicrobial agents which might be useful in the treatment of infection, and analyzes the various approaches to in vitro testing.

Influence of growth medium and supplement on growth of Haemophilus influenzae and on antibacterial activity of several antibiotics

In the present study, five non-beta-lactamase- and five beta-lactamase-producing strains of Haemophilus influenzae were used to determine whether three different growth media, Mueller-Hinton broth and agar, brain heart infusion broth and agar, and tryptic soy broth and agar, and their added supplements (0.2% hemin-0.1% IsoVitaleX, 1% hemin-1% IsoVitaleX, 2% sheep blood, 10% Fildes enrichment, 5% Fildes enrichment, 1% supplement B, 5% horse erythrocytes, and 2% hemoglobin-1% IsoVitaleX) would influence the growth rate of this microorganism and the antibacterial activity of eight antibiotics, including ampicillin, tetracycline, chloramphenicol, gentamicin, cefamandole, erythromycin, trimethoprim-sulfamethoxazole (TMP-SMX), and cefoperazone. The growth curve studies were carried out with an initial inoculum of 10(4) bacteria per ml, and MICs were determined with an inoculum of 5 X 10(5) microorganisms. Mueller-Hinton broth, brain heart infusion broth, and tryptic soy broth enriched with 5% Fildes resulted in a maximal growth of more than 10(8) CFU/ml at 24 h. When 10% Fildes or 2% sheep blood was added as enrichment to Mueller-Hinton broth, a considerable reduction in the growth rate of H. influenzae strains resulted (P less than 0.01). Significant variations in MICs (P less than 0.01) were observed with chloramphenicol, TMP-SMX, erythromycin, and cefoperazone when brain heart infusion agar, Mueller-Hinton agar, or tryptic soy agar was used. Chloramphenicol, gentamicin, erythromycin, and TMP-SMX were all affected by the different enrichments added to Mueller-Hinton agar. MICs were in general higher with 5% Fildes enrichment and lower with 1% supplement B. Cefoperazone was the only drug which exhibited a lower MIC in 5% Fildes enrichment for ampicillin-resistant H. influenzae strains.

Inoculum effect of beta-lactam antibiotics on Enterobacteriaceae

Seven beta-lactam antibiotics were studied for both their antimicrobial activity and the degree to which they produced inoculum effect on Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium. Aztreonam, cefoperazone, and ceftazidime were poorly bactericidal, caused marked bacterial filamentation, and exhibited a large inoculum effect on E. coli, K. pneumoniae, and S. typhimurium. Cefotaxime and ceftriaxone were more rapidly bactericidal, caused only a moderate amount of filamentous forms, and exhibited a modest inoculum effect, while cefoxitin and imipenem both were rapidly bactericidal and exhibited only a minimal-to-no-inoculum effect. The inoculum effect did not correlate with drug stability during incubation with the bacteria. Inoculum effect on these species of the family Enterobacteriaceae appears to be a manifestation of increase in optical density secondary to the development of filamentous bacterial forms with an increase in bacterial mass during exposure to antibiotics which are not rapidly bactericidal. These observations have a clear significance for the susceptibility testing of beta-lactam antibiotics when turbidity is used as a parameter to determine presence of bacterial growth.

Tolerance of Haemophilus influenzae to beta-lactam antibiotics

Two hundred clinical isolates of Haemophilus influenzae were tested for tolerance (MBC/MIC greater than or equal to 32) to ampicillin and cefotaxime by broth dilution tests. Of 200 strains, 9 were tolerant to ampicillin, and 10 were tolerant to cefotaxime. Tolerant organisms were identified in both systemic and nonsystemic infections and among different biotypes and serotypes of H. influenzae. These tolerant isolates were compared with nontolerant isolates by broth dilution and killing curves with log-phase and stationary-phase inocula. Both tolerant and nontolerant bacteria in log phase were killed more rapidly by antibiotics than bacteria in stationary-phase growth. When tested against 11 different beta-lactams, several patterns of tolerance were observed. Six of the ten strains were tolerant to aztreonam, four were tolerant to cefuroxime, three were tolerant to cefamandole, and two were tolerant to cefoxitin. Strain H130 was tolerant to all beta-lactam antibiotics studied. None of the 10 tolerant H. influenzae isolates were tolerant to chloramphenicol, rifampin, tobramycin, ciprofloxacin, enoxacin, and trimethoprim-sulfamethoxazole. Although the clinical significance of tolerance is not determined, this study suggests that the bactericidal activity (MBC) of beta-lactam antibiotics against H. influenzae should be determined in cases of severe infections in which clinical response is slow or unsatisfactory.

Activities of newer beta-lactam antibiotics against ampicillin, chloramphenicol, or multiply-resistant Haemophilus influenzae

The susceptibilities of singly or multiply-resistant clinical isolates of Haemophilus influenzae were determined by agar dilution to aztreonam, imipenem, and six third-generation cephalosporins. These included selected isolates that were resistant to ampicillin only, chloramphenicol only, and four isolates that were resistant to ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole. All of the isolates were highly susceptible to these newer beta-lactam antibiotics. Isolates resistant to trimethoprim-sulfamethoxazole and/or chloramphenicol had susceptibilities similar to those of strains resistant only to ampicillin. Ceftriaxone, ceftizoxime, and cefotaxime were the most active of the study antibiotics (MIC90 = 0.004-0.016 micrograms/ml), and were also bactericidal at concentrations no more than twice the minimum inhibitory concentration (MIC). Minimum inhibitory concentrations of cefoperazone increased dramatically with only a 10-fold increase in inoculum size of beta-lactamase producing strains, while MICs of the other new agents were not significantly affected by elevation of the inoculum. These new antibiotics appear to be promising candidates for therapy of infections due to resistant H. influenzae.

Third-generation and investigational cephalosporins: II. Microbiologic review and clinical summaries

In vitro susceptibility of Streptococcus pyogenes, Staphylococcus aureus, Staphylococcus epidermidis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Serratia marcescens, Hemophilus influenzae, Bacteroides fragilis, and Neisseria gonorrhea to three new second-generation and eight third-generation cephalosporins is tabulated. In general, the newer cephalosporins have an extended spectrum of activity against gram-negative bacteria, including Serratia marcescens, Pseudomonas aeruginosa, and Neisseria gonorrhea. They also tend to be active against anaerobes, including Bacteroides fragilis. However, they generally have less activity against gram-positive bacteria when compared with the first- and second-generation cephalosporins. Clinical summaries are given for each of the cephalosporins, with emphasis on the results of comparative clinical trials. These cephalosporins may prove especially useful in nosocomial infections with resistant organisms, intraabdominal infections, febrile episodes in the granulocytopenic patient, and meningitis.

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.

In vitro activity of ceftazidime (GR 20263) and other beta-lactam antibiotics against Haemophilus influenzae

Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) were determined for ceftazidime (GR 20263 - pentahydrate), RO 13-9904, cefoperazone, cefotaxime, lamoxactam, cefamandole and ampicillin using clinical isolates of beta-lactamase-producing and beta-lactamase-negative Haemophilus influenzae. RO 13-9904 and cefotaxime were the most active, followed by ceftazidime, lamoxactam and cefoperazone. Ceftazidime displayed high stability against the beta-lactamase-producing strains with a low MBC:MIC ratio.

Antimicrobial activity, pharmacokinetics, adverse reactions, and therapeutic indications of cefoperazone

Cefoperazone is a parenteral cephalosporin antibiotic that is pending approval by the U. S. Food and Drug Administration. Compared to most other cephalosporins cefoperazone has a greatly expanded spectrum of bactericidal activity that encompasses most aerobic gram-positive bacteria except enterococci, most aerobic gram-negative bacteria, including a majority of Pseudomonas aeruginosa strains, and a number of pathogenic anaerobic bacteria. Its long serum half-life, approximately two hours, permits a twelve hourly dosing schedule. No dosage modification is required in patients with renal insufficiency, and only minor modification is needed in patients with hepatic insufficiency or biliary obstruction. Clinical trials have established cefoperazone's efficacy in lower respiratory tract infections, urinary tract infections, and a variety of other bacterial infections. Adverse reactions have been infrequent, and few serious reactions have been identified. Cefoperazone is a promising new agent for the treatment of gram-negative bacillary and polymicrobial infections, especially in settings that require empiric therapy.

In vitro activity of moxalactam alone and in combination with penicillin against common meningeal pathogens

Moxalactam demonstrated marked activity against beta-lactamase-positive and -negative Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis by both standard minimal inhibitory concentration testing and growth curve studies. Moxalactam was ineffective against S. pneumoniae partially susceptible to penicillin G. Moxalactam (5 micrograms/ml) and penicillin (1 microgram/ml) in combination were indifferent to each other's antibacterial activity, exerting neither synergism nor antagonism against these organisms.

Cefoperazone: A review of its in vitro antimicrobial activity, pharmacological properties and therapeutic efficacy

Cefoperazone is a new 'third generation' semisynthetic injectable cephalosporin. It has a broad spectrum of antibacterial activity which includes Pseudomonas aeruginosa (unlike older cephalosporins), along with the Enterobacteriaceae and other Gram-negative bacteria, Gram-positive bacteria and anaerobic bacteria. Unlike other cephalosporins, cefoperazone is excreted to a considerable extent by extrarenal mechanisms. Given by intravenous or intramuscular injection, cefoperazone is effective against a wide variety of infections caused by Gram-negative or Gram-positive aerobes, including infections of the biliary tract, and in many anaerobic infections. It is generally well tolerated, diarrhoea and skin rashes being the most frequently reported side effects.

Bactericidal activity of cefoperazone with CP-45,899 against large inocula of beta-lactamase-producing Haemophilus influenzae

Bactericidal activity of cefoperazone, alone and in combination with the beta-lactamase inhibitor CP-45,899, was tested against inocula of 10(7) colony-forming units of beta-lactamase-producing isolates of Haemophilus influenzae type b per ml. Of 19 strains tested, 10 required greater than or equal to 64 microgram of cefoperazone per ml for killing, whereas no strains were killed by less than 64 microgram of CP-45,899 per ml. Synergy occurred with the combination of 4 microgram of each agent per ml against 9 of the 10 cefoperazone-resistant strains.