Microbiological investigation of cephalosporins

Subinhibitory antimicrobial concentrations: A review of in vitro and in vivo data

Antimicrobial activity is not an 'all or none' effect. An increase in the rate and extent of antimicrobial action is usually observed over a wide range of antimicrobial concentrations. Subinhibitory antimicrobial concentrations are well known to produce significant antibacterial effects, and various antimicrobials at subinhibitory concentrations have been reported to inhibit the rate of bacterial growth. Bacterial virulence may be increased or decreased by subinhibitory antimicrobial concentrations by changes in the ability of bacteria to adhere to epithelial cells or by alterations in bacterial susceptibility to host immune defences. Animal studies performed in rats, hamsters and rabbits demonstrate decreased bacterial adherence, reduced infectivity and increased survival of animals treated with subinhibitory antimicrobial concentrations compared to untreated controls. The major future role of investigation of subinhibitory antimicrobial concentrations will be to define more fully, at a molecular level, how antimicrobials exert their antibacterial effects.

Prophylaxis with oral cefadroxil versus intravenous cefuroxime in trochanteric fracture surgery. A clinical multicentre study

A prospective, randomized study was performed in 559 patients to compare two doses of oral cefadroxil with three doses of intravenous cefuorxime as antibiotic prophylaxis in intra- and subtrochanteric hip fracture surgery. Antibiotic concentrations in the wound fluid were determined at the start and at the end of the operation. The first dose of cefadroxil was given about 2 h before surgery and cefuroxime about 30 min before operation. In 226/242 (93%) patients randomized to oral cefadroxil, the concentration in the wound during surgery was on average 15 micrograms/ml, i.e., well above the minimum inhibitory concentration (MIC-90) for Staphylococcus aureus. In the cefuroxime group, antibiotic levels in the wound exceeded the MIC-90 for S. aureus in 204/210 (97%) of the patients at the start and/or at the end of surgery. All patients were followed up for 4 months. One deep and five superficial infections occurred in the cefuroxime group and no deep but one superficial infection in the cefadroxil group (P = 0.07). S. aureus was cultured in three of the infected cases while cultures were negative in four patients. Four of the seven infected patients had adequate levels of antibiotic in the wound during surgery, and in three patients no antibiotic assay was performed. The infected patients did not differ in age, sex, operation time, bleeding or any other basic variable compared with patients who healed without complications. Two doses of cefadroxil seems to be practical and as effective as intravenously administered cefuroxime as antibiotic prophylaxis in trochanteric hip fracture surgery.

Randomised double-blind study of norfloxacin and cefadroxil in the treatment of acute pyelonephritis

In a coordinated, double-blind multi-centre trial, adults with symptoms of acute pyelonephritis were randomly assigned to receive a two-week course of oral treatment with either 400 mg norfloxacin twice daily or 1 g cefadroxil twice daily. Of 197 patients enrolled in the study, 140 could be evaluated for drug efficacy and 193 for drug safety. Norfloxacin gave a significantly higher bacteriological cure rate than cefadroxil, both at 3 to 10 days (98% versus 65%; p less than 0.0001; 95% confidence interval (CI) for difference in proportions 21-46%) and up to eight weeks (87% versus 48%; p less than 0.0001; 95% CI 25-54%) after cessation of treatment. The differences between the two regimens were most pronounced in men and in patients with complicating factors such as diabetes mellitus and urinary tract abnormalities. The clinical response during treatment did not differ between the two groups, but symptomatic recurrences at follow-up were more common in the cefadroxil group (28% versus 3%; p less than 0.0001; 95% CI 14-36%). Adverse events were more often reported by patients receiving cefadroxil (39% versus 22%; p = 0.011; 95% CI 4-30%) and consisted mainly of gastrointestinal disturbances and vulvo-vaginitis. In terms of bacteriological and clinical efficacy and safety, a two-week course of norfloxacin was superior to a two-week course of cefadroxil for oral treatment of community-acquired acute pyelonephritis.

Cephalosporins in gram-positive infections

Infections caused by Gram-positive bacteria are an important and common cause of morbidity and mortality. Staphylococci and streptococci are the most frequent infecting organisms in skin and soft tissue infections, pneumonia, bone and joint infections, and endocarditis. Anaerobic Gram-positive bacteria such as Clostridia spp. cause infections that can rapidly produce tissue necrosis and death. The cephalosporins are indicated for the treatment of infections caused by Gram-positive bacteria in certain circumstances. These include selected patients with endocarditis, osteomyelitis, septic arthritis and cellulitis. They are also used as alternatives to the penicillins in penicillin-allergic patients and for 'mixed' infections caused by Gram-positive and Gram-negative organisms. This article discusses the indications for the cephalosporin group of antibiotics in patients suffering from infections caused by Gram-positive bacteria.

Mechanisms of resistance to cephalosporin antibiotics

Cephalosporins, like other beta-lactams, bind to the bacterial penicillin-binding proteins (PBPs). These correspond to the D-ala-D-ala trans-, carboxy- and endo-peptidases responsible for catalysing the cross-linking of newly formed peptidoglycan. Resistance arises when the PBPs-and particularly the transpeptidases-are modified, or when they are protected by beta-lactamases or 'permeability barriers'. Target-mediated cephalosporin resistance can involve either reduced affinity of an existing PBP component, or the acquisition of a supplementary beta-lactam-insensitive PBP. beta-lactamases are produced widely by bacteria and may be determined by chromosomal or plasmid DNA. The chromosomal beta-lactamases are species-specific, but can be classified into a few broad groups. The plasmid-mediated enzymes cross interspecific and intergeneric boundaries. The level of beta-lactamase-mediated resistance relates to the amount of enzyme produced with or without induction; to the location of the enzyme (extracellular for Gram-positive organisms and periplasmic in Gram-negative ones); and to the kinetics of the enzyme's activity. In Gram-positive organisms the PBPs are located on the outer aspect of the cytoplasmic membrane and so shielding by permeability barriers is minimal. In Gram-negative cells, however, the PBPs are protected by the outer membrane, which most beta-lactams cross by diffusion through aqueous pores composed of 'porin' proteins. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to drug entry. Such relationships are less clear for Pseudomonas aeruginosa, where the cell may contain additional barriers between the outer membrane and the PBPs. Although elevated cephalosporin resistance often is attributed to a single factor (PBP-modification, beta-lactamase action or impermeability) an organism's response to a drug often reflects the interplay of several factors. Mathematical models can be proposed to describe this interplay.

Role of cephalosporins in gonorrhoea and other sexually transmitted diseases

Cephalosporins have a role in the treatment of gonorrhoea, and especially infections caused by strains that are penicillin-resistant, either because they produce plasmid-mediated beta-lactamase or they have chromosomally mediated diminished permeability or modified penicillin-binding proteins. Although none of the oral or Group I agents are useful, most of the Group II, III and IV agents are, and especially cefuroxime, cefotaxime, ceftriaxone and cefoxitin. In addition to uncomplicated urethral, cervical or rectal infections, appropriate regimens are also effective for the treatment of pharyngeal infections, disseminated infections and gonococcal ophthalmia. The cephalosporins have no clear role in the treatment of syphilis, granuloma inguinale, Mycoplasma or chlamydial infections or bacterial vaginosis, but ceftriaxone may be effective in chancroid, and cefoxitin in combination with an antichlamydial agent (such as a tetracycline) might be used for the treatment of pelvic inflammatory disease.