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

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.

Pharmacodynamic (kinetic) considerations in the treatment of moderately severe infections with cefotaxime

Information about the pharmacodynamics of beta-lactams has accumulated rapidly over the last 20 years, and their application to cefotaxime are discussed in this review. Application of pharmacodynamics requires an integration of the pharmacokinetic and in vitro properties of the agent. Cefotaxime is similar to other beta-lactams in that it has little concentration-dependent killing and produces no postantibiotic effect against Gram-negative bacteria. However, it has a microbiologically active metabolite, deascetylcefotaxime, which can show synergy, partial synergy, or an additive effect in combination with the parent drug. More than any other technique, animal models have been able to elucidate the pharmacokinetic parameters that predict efficacy in vivo. They have shown that for beta-lactams it is the time that levels exceed the minimum inhibitory concentration (MIC) that is the most important determinant of efficacy. For bacteria to have no postantibiotic effect, plasma levels need to exceed the MIC for the whole of the dosing interval to achieve maximum killing at the site of infection. When applying these concepts as the most stringent criteria for efficacy using pharmacokinetic values from young, healthy volunteers, it can be shown that organisms with MICs of < or = 0.03 microgram/ml for a 1-g dose and 0.06 microgram/ml for a 2-g dose to achieve optimum efficacy with 12-h dosing of cefotaxime. However, two clinical studies have demonstrated trough levels much greater than would be predicted from these pharmacokinetic values, as a result of the effects of decreased renal function accompanying sepsis and older age. These studies showed that organisms with MICs < or = 1 microgram/ml for a 1-g dose or 2 micrograms/ml for a 2-g 12-h dose were covered for the whole of the dosing interval. Thus, all strains of Enterobacteriaceae and pathogenic Neisseria spp. that lack resistance mechanisms to third-generation cephalosporins would be covered using 12-h dosing schedules.

Cerebrospinal fluid penetration of amikacin in children with community-acquired bacterial meningitis

The penetration of amikacin into the cerebrospinal fluid (CSF) was studied with 16 children (mean age, 1 year and 9 months; range, 4 months to 8 years) with community-acquired bacterial meningitis. Amikacin was given intravenously at a dose of 7.5 mg/kg of body weight twice daily. CSF was collected on day 1, at the expected peak concentration of amikacin in CSF. The mean (standard deviation) concentration of amikacin in CSF was 1.65 (1.6) mg/liter. Concentrations of amikacin in CSF correlated significantly with CSF glucose levels on admission. The mean concentrations of amikacin in CSF were 2.9, 1.1, and 0.20 mg/liter in patients with CSF glucose levels of < 1, 1 to 2, and > 2 mmol/liter, respectively. Thus, amikacin penetrates the blood-brain barrier substantially in children with bacterial meningitis and achieves particularly high concentrations when CSF glucose level is < 1 mmol/liter on admission.

Bacterial counts in cerebrospinal fluid of children with meningitis

Eighty-five cerebrospinal fluid (CSF) specimens from the same number of pediatric patients with meningitis were examined to determine the bacterial count and the relationship of this count to the microscopy results, the ages of the patients and the bacterial species isolated. Bacterial counts ranged from 2 x 10 to 4 x 10(9) CFU/ml CSF. Twenty-five percent of the 85 CSF specimens positive for Haemophilus influenzae type b, Neisseria meningitidis, Streptococcus pneumoniae, Escherichia coli K1 and group B streptococci had counts of 10(7) CFU/ml or higher. Children between 1 and 6 months of age had significantly higher counts (p less than 0.05) than the other age groups. The three patients who had positive CSF cultures 24 h after the start of therapy all had initial bacterial counts of 10(7) CFU/ml or higher. The detection limit for Gram stain/microscopy was 10(5) CFU/ml. No correlation was found between bacterial count and the number of polymorphonuclear leukocytes.

In vitro synergism of rifampin-cephalosporin combinations against Haemophilus influenzae type b

The in vitro susceptibility of Haemophilus influenzae type b to rifampin, cefotaxime, ceftazidime, ceftriaxone and cefuroxime was examined at inocula of 10(5) and 10(7) CFU/ml. Time-kill curves were then obtained using antibiotic concentrations at one-half the MIC for each drug at the two inocula with combinations of rifampin plus each of the cephalosporins. There was a pronounced inoculum effect with all of the cephalosporins except for cefuroxime, but the MIC values were also higher for the latter drug. The rare failure of some cephalosporins to promptly sterilize the cerebrospinal fluid in meningitis should encourage investigation of the relationship between the inoculum effect, minimum bactericidal concentration and cerebrospinal fluid antibiotic levels in these clinical situations. A marked synergistic effect was noted for most of the isolates, and therefore, cephalosporin-rifampin combinations might show clinical utility.

Comparative efficacy of ceftriaxone and cefuroxime for treatment of bacterial meningitis

To assess the comparative efficacy of cefuroxime and ceftriaxone for the treatment of bacterial meningitis, we reviewed the records from four prospective efficacy trials conducted at our institution. One hundred seventy-four infants and children received ceftriaxone and 159 received cefuroxime. The clinical characteristics of the two groups were comparable at admission. After 24 hours of therapy, routine cerebrospinal fluid cultures for all patients treated with ceftriaxone were sterile, whereas 9% of cerebrospinal fluid cultures were positive in cefuroxime-treated patients (p less than 0.001). More cefuroxime-treated patients had abnormal physical examinations at the time of discharge than did ceftriaxone-treated patients (39/159 vs 25/174, p = 0.02). At 6-week and 1-year follow-up examinations, there was no longer a statistically significant difference in the incidence of neurologic abnormalities between the two therapy groups, but the incidence of hearing impairment in one or both ears was higher in the cefuroxime (18%) than in the ceftriaxone (11%) treatment group. Both regimens are efficacious for the treatment of bacterial meningitis, but some patients may not respond as satisfactorily to cefuroxime as to ceftriaxone.

Mechanisms of beta-lactam resistance in Haemophilus influenzae

Haemophilus influenzae has become increasingly resistant to beta-lactam antibiotics. Three major mechanisms, both enzymatic and non-enzymatic, are involved. Enzymatic resistance is mainly due to production of a TEM-1 plasmid-mediated beta-lactamase, and in some cases to a new enzyme ROB-1. Of the non-enzymatic mechanisms, decreased permeability due to alteration of outer membrane proteins seems to be rare in comparison to decreased affinity of penicillin-binding proteins for beta-lactam antibiotics. Enzymatic resistance is present in about 10-20% of clinical isolates, while non-enzymatic resistance is present only in 2-4%.

Cefmenoxime. A review of its antibacterial activity, pharmacokinetic properties and therapeutic use

Cefmenoxime is an aminothiazolyl cephalosporin administered intravenously or intramuscularly. Like other 'third-generation' cephalosporins it is active in vitro against most common Gram-positive and Gram-negative pathogens, is a potent inhibitor of Enterobacteriaceae (including beta-lactamase-producing strains), and is resistant to hydrolysis by beta-lactamases. Cefmenoxime has a high rate of clinical efficacy in many types of infection and is at least equal in clinical and bacteriological efficacy to several other cephalosporins in urinary tract infections, respiratory tract infections, postoperative infections and gonorrhoea. Cefmenoxime, like latamoxef, cefoperazone and cefamandole, has an N-methyltetrazole side chain at the 3-position of the cephalosporin nucleus and thus possesses the potential for producing hypoprothrombinaemic bleeding and disulfiram-like reactions. However, these reactions have been reported very rarely and the antibacterial is generally well tolerated. It is likely that cefmenoxime will most closely resemble cefotaxime and ceftizoxime in therapeutic profile and usefulness.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.

Pericarditis caused by beta-lactamase-producing Haemophilus influenzae: report of two cases in adults and review of the literature

Two adult patients with pericarditis caused by beta-lactamase producing Haemophilus influenzae are reported and their management reviewed. Both had pharyngitis, epiglottitis, pneumonia, empyema, or septicemia and were cured with antimicrobics and pericardial drainage (one by catheter and one by surgery). Eleven previously reported cases of pericarditis caused by Haemophilus influenzae are also reviewed. In reviewing this rare cause of bacteria pericarditis, it is important to recognize the antibiotic resistance profile, the incidence of pericardial tamponade, and the use of surgical drainage. Antibiotic selection for this organism is also discussed, as well as the importance of biotyping.

Cefuroxime versus ampicillin plus chloramphenicol in childhood bacterial meningitis: a multicenter randomized controlled trial

In a multicenter randomized trial, 107 children with bacterial meningitis were initially given either cefuroxime or ampicillin plus chloramphenicol. Patients were alternately assigned to 7- or 10-day courses of the designated antimicrobial regimen. CSF isolates included Haemophilus influenzae type b (89, of which 25% were beta-lactamase positive), Streptococcus pneumoniae, and Neisseria meningitidis. Although mean CSF bactericidal titers against Haemophilus isolates were 1:6 in each treatment group, H. influenzae was cultured from CSF in four of 39 patients receiving cefuroxime, 24 to 48 hours after initiation of therapy, compared with none of 40 patients given ampicillin plus chloramphenicol (P = 0.11). Clinical cure rates were similar (95%); one death occurred in each group. One child given cefuroxime had persistent meningitis after 5 days of therapy, and mastoiditis with secondary bacteremia developed in one on day 10. Three patients had relapse or reinfection. One patient who received cefuroxime for 10 days had a relapse of epiglottitis 17 days later, and of the patients given ampicillin plus chloramphenicol, one had a relapse of meningitis 1 week after 7 days of therapy, and bacteremia developed in one 42 days after completion of 10 days of therapy. No increase in either in-hospital complications or relapses occurred with a 7-day treatment course. Proof of the equivalence of the antibiotic regimens and the efficacy of 7-day courses of treatment, as well as the consequences of delayed CSF sterilization, will require additional investigation.

Ceftriaxone: a beta-lactamase-stable, broad-spectrum cephalosporin with an extended half-life

Ceftriaxone is an aminothiazolyl-oxyimino cephalosporin. It possesses the typical in vitro activity of a third-generation cephalosporin with excellent activity against many gram-negative aerobic bacilli: Escherichia coli; species of Proteus, Klebsiella, Morganella, Providencia and Citrobacter; and Enterobacter agglomerans. Ceftriaxone also has outstanding bactericidal action against pneumococci, group B streptococci, meningococci, gonococci and Hemophilus influenzae. In healthy volunteers, it has an exceptionally long serum half-life of 5.8-8.7 (mean 6.5) hours. It distributes well throughout all body spaces, including cerebrospinal fluid in the presence of inflammation. Dosage modification is necessary only when there is combined hepatic and renal dysfunction. Adverse reactions characteristic of cephalosporins have been observed with the administration of ceftriaxone. No unique toxicities have been identified, and hypoprothrombinemic bleeding is not part of the adverse reaction profile. Ceftriaxone has been used to treat serious bacterial infections in neonates, infants, children and adults. Bacteriologic and clinical success rates have consistently exceeded 90%. The drug has also been used as single-dose chemoprophylaxis in coronary artery bypass, biliary tract, vaginal hysterectomy and prostatic surgery. Efficacy and safety were similar to multiple-dose cefazolin. Ceftriaxone warrants special consideration because its extended half-life allows for less frequent dosing than other antimicrobials. Significant cost savings can be realized with proper use of this antibiotic.

Bacterial meningitis in infants and children

Bacterial meningitis of infants and children exact a major toll worldwide. Haemophilus influenzae, Streptococcus pneumoniae and Neisseria meningitidis continue to be the major causes, and efforts to reduce the incidence of this disease have had limited success. Major advances in rapid diagnosis, such as antigen detection, have occurred over the past several decades; however, Gram stain, cytologic examination and culture of the CSF remain the most reliable means of making an etiologic diagnosis. Despite shifts in bacterial susceptibilities to commonly used antibacterial agents, penicillin, ampicillin and chloramphenicol remain effective therapy for the vast majority of cases, although newer cephalosporin derivatives may be required to avoid the toxicities of chloramphenicol and to effectively treat selected bacteria that have developed resistance to these more classical antibiotics. The prognosis for bacterial meningitis in infants and children remains guarded because of late diagnosis and the severity of the disease. Major reductions in the incidence of the disease will depend on definition of high-risk populations and application of appropriate preventions.