Unusual nosocomial infections

Resistance of Xanthomonas maltophilia to antibiotics and the effect of beta-lactamase inhibitors

We examined the susceptibility of 50 isolates of Xanthomonas maltophilia and the effect of beta-lactamase inhibitors upon the susceptibility. The majority of isolates were resistant to azlocillin, piperacillin, mezlocillin, ticarcillin, cefotaxime, ceftizoxime, ceftriaxone, cefoperazone, and ceftazidime. All isolates were resistant to imipenem, CGP 31608, aztreonam, and carumonam. Although disk susceptibility tests showed that the combination of clavulanate with ticarcillin inhibited many isolates, at a ratio of 1:20 few isolates were susceptible to the combination. Addition of clavulanate to aztreonam and to imipenem failed to make organisms susceptible. Sulbactam combined with cefoperazone made some organisms susceptible, but ampicillin-sulbactam was ineffective, whereas tazobactam combined with piperacillin at a ratio of 1:4 made half the isolates have MICs of 32 micrograms/ml or less. The beta-lactamases from the isolates hydrolyzed all of the beta-lactams.

Cephalosporins in the treatment of meningitis

The synthesis of new cephalosporin antibiotics has provided agents which can effectively be used to treat most of the different forms of meningitis. None of the first generation cephalosporins can be considered acceptable as agents to treat meningitis. Cefuroxime can be used to treat meningitis due to Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis in children. Agents such as cefotaxime and ceftriaxone are appropriate for neonatal meningitis due to Escherichia coli and group B streptococci, but not Listeria monocytogenes. Cefotaxime, ceftriaxone, ceftizoxime and ceftazidime have all proved effective as therapy of meningitis in children and adults when the pathogens are pneumococci, H. influenzae or N. meningitidis, but they have not been shown to yield an improved mortality or lower morbidity in spite of much greater cerebrospinal fluid (CSF) bactericidal titres. Cefotaxime, ceftizoxime, ceftriaxone and ceftazidime have been effective as therapy of meningitis due to E. coli, K. pneumoniae and Proteus species, but failures have occurred with all of the cephalosporins when used to treat meningitis due to Enterobacter spp. and Serratia marcescens. Only ceftazidime yields adequate CSF concentrations to treat meningitis due to Pseudomonas aeruginosa. Overall, the cephalosporins can now be considered a major component of the therapy of acute bacterial meningitis irrespective of the age group to be treated.

Relation of structural properties of beta-lactam antibiotics to antibacterial activity

There has been remarkable progress in the development of new antimicrobial agents as the result of structural modifications of the cephalosporin nucleus. It has been possible to predict many aspects of the antimicrobial activity of new agents and to recognize the structural modifications that contribute to overcoming the continued problem of bacterial resistance. The activity of beta-lactams against gram-positive species depends primarily on their affinity for the enzymes referred to as penicillin-binding proteins. Resistance of gram-positive species to beta-lactams is either due to altered penicillin-binding proteins or, more commonly, due to the presence of beta-lactamases, which are usually plasmid-mediated and inducible. The activity of beta-lactams against gram-negative aerobic and anaerobic bacteria is the result of the way in which the compounds pass through the porin channels in the outer wall, resist inactivation by beta-lactamases, and bind to the penicillin-binding proteins. The basic cephalosporin nucleus consists of the essential beta-lactam ring fused to a dihydrothiazine ring. It is possible to modify this structure to increase antibacterial activity. Changes in moieties at position 3 affect pharmacologic activity but can also cause a marked increase or decrease in activity against staphylococci and Pseudomonas species. The presence of the thiomethyltetrazole group at position 3 has been associated with an alteration in prothrombin synthesis and with disulfiram reactions. Modifications of the cephem nucleus at position 7 by addition of methoxy groups increase beta-lactamase stability but decrease activity against gram-positive species because of lower affinity for penicillin-binding proteins. The more useful acyl side chains have been those that contain a 2-aminothiazolyl moiety, which causes increased affinity of the molecules for penicillin-binding proteins of gram-negative bacteria and streptococcal species. Iminomethoxy groups provide beta-lactamase stability against the common plasmid beta-lactamases such as those of Staphylococcus aureus and the TEM, SHV-1, OXA, and PSE enzymes found in Enterobacteriaceae and Pseudomonas aeruginosa, as well as the chromosomally mediated K-1 and P99 enzymes of Enterobacter. A propylcarboxy group increases beta-lactamases stability and also provides activity against P. aeruginosa and some Acinetobacter. Conversely, this particular grouping reduces the beta-lactamase induction capabilities of a compound, as well as its ability to function as a beta-lactamase inhibitor.(ABSTRACT TRUNCATED AT 400 WORDS)