Changes in antibiotic sensitivity patterns of Gram-negative bacilli in burns

Effects of rate of infusion and probenecid on serum levels, renal excretion, and tolerance of intravenous doses of cefoxitin in humans: comparison with cephalothin

Using a randomized crossover design, 1-g intravenous doses of cephalothin and cefoxitin, a cephalosporinase-resistant cephamycin, were infused into 12 normal adult males over periods of 120, 30, and 3 min, the last with and without prior intravenous infusions of probenecid (1 g). Mean peak serum concentrations of antibiotic activity after cephalothin infusions were 23, 56, 103, and 102 mug/ml, respectively, and after cefoxitin infusions they were 27, 74, 115, and 125 mug/ml, respectively. Probenecid treatment prolonged the terminal serum half-life of cephalothin-like activity from 0.52 to 1.0 h, and of cefoxitin from 0.68 to 1.4 h. In contrast to cephalothin, which was found to be metabolized about 25% to the less active desacetyl form, cefoxitin was metabolized less than 2% to the virtually inactive descarbamyl form, as judged from urinary recoveries. Neither antibiotic displayed detectable organ toxicity. Of 300 recent clinical isolates of gram-negative bacilli other than Pseudomonas spp., 83% were susceptible to cephalothin but 95% were susceptible to cefoxitin. Organisms resistant to cephalothin but susceptible to cefoxitin included strains of Escherichia coli, Proteus vulgaris, Klebsiella spp., Serratia marcescens, Enterobacter spp., and Bacteroides spp.

Increasing threat of Gram-negative bacteria

The widespread use of broad-spectrum antibiotics has led to emergence of antibiotic-resistant strains of many Gram-negative organisms. This problem is particularly serious in critically ill patients, especially those with ventilator-associated pneumonia. Extensive antibiotic resistance has developed in Gram-negative bacteria, due both to innate resistance in some species and the fact that they are highly adept at acquiring antibiotic-resistant determinants from each other. Antibiotic resistance develops through the following three basic mechanisms: alteration of the drug target, prevention of drug access to the target (including actively removing the drug from the bacteria), and drug inactivation. Certain Gram-negative microorganisms are particular problems in the intensive care unit, including Pseudomonas aeruginosa, Acinetobacter spp., Stenotrophomonas maltophilia, and the Enterobacteriaceae. The combination of an increasing population at risk, and the natural virulence and adaptability of Gram-negative bacteria guarantees that critical care physicians will face a persistent and increasing challenge from these pathogens.

Novel and emerging mechanisms of antimicrobial resistance in nosocomial pathogens

Nosocomial pathogens frequently are resistant to antimicrobial agents. Although methicillin-resistant strains of Staphylococcus aureus continue to be a major problem in many hospitals, several new types of resistance determinants have been noted among organisms causing hospital-acquired infections. The mechanisms include extended spectrum beta-lactamases in gram-negative bacilli; resistance to beta-lactams, glycopeptides, and high levels of aminoglycosides among enterococci; quinolone resistance in isolates of methicillin-resistant S. aureus; and the spread of multiple resistance genes simultaneously in gram-negative organisms via Tn21-related genetic elements. These novel mechanisms of resistance complicate the treatment of nosocomial infections by limiting the number of effective antimicrobial agents available to the clinician. It is important for infection control practitioners and microbiologists to work together to detect and control the spread of resistant pathogens in the hospital setting.

Gentamicin- and silver-resistant pseudomonas in a burns unit

In 1977-8 gentamicin-resistant strains of Pseudomonas aeruginosa became very common in a burns unit, over 90% being resistant at the peak of the outbreak. Some strains were also resistant to silver nitrate, though silver resistance was not found in any other strains of Ps aeruginosa isolated. Unlike the gentamicin resistance, the silver resistance was unstable, and strains became sensitive on repeated subculture. All the gentamicin-resistant strains of Ps aeruginosa were of the same serotype (O:11, H:2,5). Though gentamicin resistance could be transferred in vitro from resistant strains of Ps aeruginosa to one sensitive strain of Ps aeruginosa, there was no evidence of in-vivo transfer of gentamicin resistance between strains of pseudomonas in the patients' burns, nor was there evidence of transfer of gentamicin resistance between Ps aeruginosa and enterobacteria. Carbenicillin-resistant and gentamicin-resistant Ps aeruginosa were sometimes found in the same burns, but no gentamicin-carbenicillin (doubly) resistant strains were found among the 986 strains tested during the outbreak. The outbreak of gentamicin-resistant Ps aeruginosa from burns was not reduced by stopping treatment with gentamicin and its analogues but only by segregating all patients with Ps aeruginosa in one of the two wards of the unit and admitting new patients only to the other ward.

Chloramphenicol resistance plasmids in Escherichia coli isolated from diseased piglets

The plasmids in 19 chloramphenicol-resistant Escherichia coli strains of three pig pathogenic antigen types were studied in conjugation and transduction experiments. The plasmids had identical resistance patterns: streptomycin, spectinomycin, sulfonamides, and chloramphenicol (Sm, Sp, Su, Cm) and belonged to IncFII. One plasmid carried ampicillin resistance in addition. Restriction enzyme analysis of the deoxyribonucleic acid from five of the plasmids originating from the same herd showed that their digestion patterns with EcoRI were indistinguishable. EcoRI cleaved the deoxyribonucleic acid of a sixth plasmid from the same herd and displayed nine of the ten bands of the other five plasmids plus an additional six. It appears that the five plasmids with identical restriction patterns have a common origin and may be copies of the same plasmid from which the sixth may have developed. Four strains carried two plasmids each. In two of these strains, a plasmid with a tetracycline marker (Tc), or possibly the tetracycline marker alone, recombined frequently with the Sm Sp Su Cm plasmid without destroying any known function of the latter. The possibility that Tc is carried on a translocation sequence is discussed.

Drug resistance in relation to use of silver sulphadiazine cream in a burns unit

Topical chemoprophylaxis of extensive burns with silver sulphadiazine cream led to a large increase in the proportion of sulphadiazine-resistant Gram-negative bacilli in a burns unit. When all sulphonamide treatment in the ward was stopped; the incidence of sulphonamide-resistant strains fell back to levels similar to those recorded when silver sulphadiazine treatment was introduced. This was associated with a large reduction in the incidence of resistance of certain Gram-negative bacilli (especially Klebstella sp) to several antibiotics. Transferable resistance to sulphadiazine, shown by conjugation experiments with Escherichia coli K12, was found in a majority of the strains of Klebsiella sp tested, and in other species. A pattern of transferable resistance to tetracycline, cephaloridine, chloramphenicol, ampicillin, carbenicillin, and sulphadiazine (T Ce Cl A Ca S) was found in four of the 22 strains of Klebsiella tested, and closely related patterns were transferred by five other strains. These patterns of resistance were commonly found in Klebsiella sp isolated from burns in the period before the withdrawal of sulphonamides from the ward but were found in none of the Klebsiella strains isolated in the first six months after that period. Strains of Acinetobacter and Proteus, in which transferable resistance was not found, showed no appreciable fall or rise in sulphadiazine resistance; there was no fall in resistance of these organisms to tetracycline, cephaloridine, chloramphenicol, ampicillin or carbenicillin on withdrawal of sulphonamides from the ward, but there were substantial falls in resistance of Acinetobacter to kanamycin, gentamicin, trimethoprim, and tetracycline which were probably not caused by the withdrawal of sulphonamides.

Tobramycin, amikacin, sissomicin, and gentamicin resistant Gram-negative rods

Sensitivities to gentamicin, sissomicin, tobramycin, and amikacin were compared in 196 gentamicin-resistant Gram-negative rods and in 212 similar organisms sensitive to gentamicin, mainly isolated from clinical specimens. Amikacin was the aminoglycoside most active against gentamicin-resistant organisms, Pseudomonas aeruginosa, klebsiella spp, Escherichia coli, Proteus spp, Providencia spp, and Citrobacter spp being particularly susceptible. Most of the gentamicin-resistant organisms were isolated from the urine of patients undergoing surgery. Gentamicin was the most active antibiotic against gentamicin-sensitive E coli, Proteus mirabilis, and Serratia spp. Pseudomonas aeruginosa and other Pseudomonas spp were most susceptible to tobramycin.