Characterization of impermeability variants of Pseudomonas aeruginosa isolated during unsuccessful therapy of experimental endocarditis

Yeast petites and small colony variants: for everything there is a season

The yeast petite mutant was first found in the yeast Saccharomyces cerevisiae. The colony is small because of a block in the aerobic respiratory chain pathway, which generates ATP. The petite yeasts are thus unable to grow on nonfermentable carbon sources (such as glycerol or ethanol), and form small anaerobic-sized colonies when grown in the presence of fermentable carbon sources (such as glucose). The petite phenotype results from mutations in the mitochondrial genome, loss of mitochondria, or mutations in the host cell genome. The latter mutations affect nuclear-encoded genes involved in oxidative phosphorylation and these mutants are termed neutral petites. They all produce wild-type progeny when crossed with a wild-type strain. The staphylococcal small colony variant (SCV) is a slow-growing mutant that typically exhibits the loss of many phenotypic characteristics and pathogenic traits. SCVs are mostly small, nonpigmented, and nonhaemolytic. Their small size is often due to an inability to synthesize electron transport chain components and so cannot generate ATP by oxidative phosphorylation. Evidence suggests that they are responsible for persistent and/or recurrent infections. This chapter compares the physiological and genetic basis of the petite mutants and SCVs. The review focuses principally on two representatives, the eukaryote S. cerevisiae and the prokaryote Staphylococcus aureus. There is, clearly, commonality in the physiological response. Interestingly, the similarity, based on their physiological states, has not been commented on previously. The finding of an overlapping physiological response that occurs across a taxonomic divide is novel.

The role of small-colony variants in failure to diagnose and treat biofilm infections in orthopedics

Biomaterial-related infection of joint replacements is the second most common cause of implant failure, with serious consequences. Chronically infected replacements cannot be treated without removal of the implant, as the biofilm mode of growth protects the bacteria against antibiotics. This review discusses biofilm formation on joint replacements and the important clinical phenomenon of small-colony variants (SCVs). These slow-growing phenotypic variants often remain undetected or are misdiagnosed using hospital microbiological analyses due to their unusual morphological appearance and biochemical reactions. In addition, SCVs make the infection difficult to eradicate. They often lead to recurrence since they respond poorly to standard antibiotic treatment and can sometimes survive intracellularly.

Small colony variants: a pathogenic form of bacteria that facilitates persistent and recurrent infections

Small colony variants constitute a slow-growing subpopulation of bacteria with distinctive phenotypic and pathogenic traits. Phenotypically, small colony variants have a slow growth rate, atypical colony morphology and unusual biochemical characteristics, making them a challenge for clinical microbiologists to identify. Clinically, small colony variants are better able to persist in mammalian cells and are less susceptible to antibiotics than their wild-type counterparts, and can cause latent or recurrent infections on emergence from the protective environment of the host cell. This Review covers the phenotypic, genetic and clinical picture associated with small colony variants, with an emphasis on staphylococci, for which the greatest amount of information is available.

Mécanisme de la résistance adaptative de Pseudomonas aeruginosa aux aminosides

Exposure of Pseudomonas aeruginosa to aminoglycosides frequently selects for recalcitrant subpopulations exhibiting an unstable, << adaptive >> resistance to these antibiotics. In this study, we investigated the implication in the phenomenon of MexXY-OprM, an active efflux system known to export aminoglycosides in P. aeruginosa. Immunoblotting experiments demonstrated that the transporter MexY, but not the outer membrane pore OprM, was overproduced during the post-drug exposure adaptation period in wild-type strain PAO1. Furthermore, MexY production was dependent upon the degree of bacterial exposure to gentamicin (drug concentration). In contrast to parental strain PAO1, mutants defective in MexXY or in OprM were unable to develop adaptive resistance. Altogether, these results indicate that the resistance process requires the rapid production of MexXY and the interaction of these proteins with the constitutively produced component OprM.

MexXY-OprM efflux pump is necessary for a adaptive resistance of Pseudomonas aeruginosa to aminoglycosides

Exposure of Pseudomonas aeruginosa to aminoglycosides frequently selects for recalcitrant subpopulations exhibiting an unstable, "adaptive" resistance to these antibiotics. In this study, we investigated the implication in the phenomenon of MexXY-OprM, an active efflux system known to export aminoglycosides in P. aeruginosa. Immunoblotting experiments demonstrated that the transporter MexY, but not the outer membrane pore OprM, was overproduced during the post-drug exposure adaptation period in wild-type strain PAO1. Furthermore, MexY production was dependent upon the degree of bacterial exposure to gentamicin (drug concentration). In contrast to parental strain PAO1, mutants defective in MexXY or in OprM were unable to develop adaptive resistance. Altogether, these results indicate that the resistance process requires the rapid production of MexXY and the interaction of these proteins with the constitutively produced component OprM.

Chronic prosthetic hip infection caused by a small-colony variant of Escherichia coli

From two different specimens of a chronic prosthetic hip infection taken at an interval of 2 months a slow-growing gram-negative bacterium was isolated in pure culture. The strain grew with the typical features of a small-colony variant (SCV). 16S rRNA sequencing identified the bacterium as Escherichia coli. Biochemical characterization demonstrated multiple phenotypic alterations of a mutant carrying a defect in the heme biosynthetic pathway (Hem-): (i) catalase and nitrate reductase reactions were both negative, (ii) a negative benzidine reaction demonstrated the lack of heme-containing cytochromes, and (iii) growth stimulation under anaerobic conditions as well as gentamicin resistance indicated defective aerobic respiration. PCR and Southern hybridization demonstrated that the mutation of the SCV of E. coli was localized in the hemB gene and was most likely due to a deletion of the hemB gene. On blood agar plates revertants were recognized growing as normal-sized colonies between the dominant small colonies of the strain. Feeding experiments indicated that the revertants but not the small colonies were permeable for hemin. A strong antibody response against the infecting SCV of E. coli was found. To our knowledge, this is the first report of a Hem- E. coli strain as the etiological agent of a chronic bacterial infection.

The effects of adherence to silicone surfaces on antibiotic susceptibility in Staphylococcus aureus

Sensitivity of Staphylococcus aureus to the antibiotics tetracycline, benzylpenicillin and vancomycin was found to decrease by 2-10 fold when cells were grown adherent to silicone catheter surfaces. Sensitivity to rifampicin and fusidic acid was not significantly altered in adherent cells. Susceptibility further decreased with increased adherence time prior to antibiotic challenge. The resistance observed was not genotypic, or due to the presence of a specialized subpopulation of bacteria, as it disappeared when the bacteria were removed from the catheter, subcultured and retested. Also, adherent bacteria were found to grow more slowly than bacteria growing planktonically. It is concluded that the decrease in antibiotic susceptibility of adherent bacteria is a function of the physiological status of the individual cells rather than a function of biofilm formation or slime production. The decrease in growth rate of the adherent bacteria is a result of the adherence process rather than a result of nutrient depletion. The decrease in growth rate is implicated, but is not the sole factor, in the decreased antibiotic susceptibility of adherent bacteria.

In vitro characterization of aminoglycoside adaptive resistance in Pseudomonas aeruginosa

Aminoglycoside adaptive resistance was characterized in one reference strain and four clinical isolates of Pseudomonas aeruginosa. Adaptive resistance was initiated with a 2-h gentamicin or tobramycin exposure at the MIC. Each P. aeruginosa strain demonstrated an adaptive-resistance period of between 8 and 12 h when tested with both aminoglycosides. Aminoglycoside adaptive resistance was shown to correlate with a decrease in [3H] gentamicin accumulation and a small (5%) but significant (P < 0.05) reduction in proton motive force. The mean generation time of P. aeruginosa during peak levels of adaptive resistance (i.e., maximum reductions in aminoglycoside killing) was not significantly different from that of control organisms (P < 0.05). No changes in outer membrane protein or lipopolysaccharide sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles were noted when control, adaptively resistant, and postadaptively resistant cells were compared. Cytoplasmic membrane profiles of adaptively resistant cells, however, demonstrated several band changes when compared with control and postadaptively resistant cells. We conclude that the decrease in aminoglycoside accumulation associated with adaptive resistance in P. aeruginosa may be, in part, a function of reductions in proton motive force and/or cytoplasmic membrane protein changes. However, the importance of these changes requires further investigation.

Interaction of gentamicin with the A band and B band lipopolysaccharides of Pseudomonas aeruginosa and its possible lethal effect

The lipopolysaccharide (LPS) of Pseudomonas aeruginosa PAO1 possesses two distinct types of O polysaccharide, A and B band LPSs, but the majority of clinical isolates from cystic fibrosis patients who are infected with the organism possess only the A band as the major LPS antigen. The initial step in a series of events during the uptake of aminoglycoside antibiotics such as gentamicin is the ionic binding of the molecule to the cell surface. In an attempt to elucidate the role of A and B band LPSs of P. aeruginosa in this passive ionic binding of gentamicin to the outer membrane and its possible lethal effects, strains PAO1 (A+B+) and LPS isogenic derivatives (A+B-,A-B+,A-B-) were treated with the antibiotic. Ionic binding of gentamicin appeared to be subtly different in PAO1 and its LPS derivatives; a lethal dose of drug was bound to all strains, although the degree of binding varied with each strain. The outer membrane affinity for gentamicin was higher in strains possessing the B band than in strains with A band LPS, and these B band strains were more prone to antibiotic-induced killing. Strains with both A and B band LPSs bound the most gentamicin of all strains, and this binding caused an almost 50% loss in viability. Ionic binding of aminoglycoside antibiotucs to the outer membrane of cell surfaces must not only weaken th cell surface (R. E. W. Hancock, Annu. Rev. Microbiol. 38:237-264, 1984; N. L. Martin and T. J. Beveridge, Antimicrob. Agents Chemother. 29:1079-1087, 1986; S. G. Walker and T. J. Beveridge, Can. J. Microbiol. 34:12-18, 1988) but it must also be more important in cell death than was originally thought.

Efficacy of ceftazidime and aztreonam alone or in combination with amikacin in experimental left-sided Pseudomonas aeruginosa endocarditis

The in vivo efficacies of ceftazidime, aztreonam, and the combinations of ceftazidime with amikacin and aztreonam with amikacin were studied in the rabbit left-sided endocarditis model by using two strains of Pseudomonas aeruginosa, one multisusceptible and one multiresistant, in a total of 156 animals. Antibiotics were given intramuscularly for 10 days, as follows: amikacin, 7 mg/kg of body weight every 8 h, and ceftazidime and aztreonam, 50 mg/kg every 8 h. All regimens except amikacin alone significantly reduced the number of CFU per gram of vegetation (P < or = 0.008), but only for the multisusceptible strain for which sterile vegetations were obtained in 20, 25, 21, 75, and 53% of the groups treated with amikacin, ceftazidime, aztreonam, and the combination groups ceftazidime-amikacin and aztreonam-amikacin, respectively (ceftazidime plus amikacin versus controls, P = 0.001). Regarding the decrease in the numbers of colonies in vegetations, (i) all regimens significantly reduced the number of CFU per gram of vegetation (P < 0.001), (ii) results with ceftazidime-amikacin compared with those with monotherapy were significantly different (P < or = 0.007), and (iii) results with aztreonam-amikacin, although better than those with monotherapy, were marginally not statistically significant. At 1 h postdose, mean amikacin, aztreonam, and ceftazidime levels in serum were 35 +/- 19.4, 89.6 +/- 8.16, and 92.61 +/- 11.52 micrograms/ml, respectively. It was concluded that the combination of ceftazidime, and possibly aztreonam, with amikacin given at high doses and short intervals could have a place in the therapy of patients with left-sided endocarditis caused by P. aeruginosa.

Liposome-encapsulated antibiotics: preparation, drug release and antimicrobial activity against Pseudomonas aeruginosa

Ticarcillin- and tobramycin-resistant strains of Pseudomonas aeruginosa were shown to have a markedly increased sensitivity to antibiotics enclosed in liposomes. This was demonstrated by growth inhibition of two resistant P. aeruginosa strains in the presence of the liposome-enclosed ticarcillin and tobramycin at 2 per cent and 20 per cent of their respective minimum inhibitory concentration. The liposome-enclosed antibiotic was as effective against the beta-lactamase-producing strain as against the non-beta-lactamase producing strain. Entrapment efficiency of the two antibiotics with the dehydration-rehydration vesicle (DRV) method was largely superior to other methodologies used. Kinetic studies with DRV demonstrated that tobramycin and ticarcillin-loaded liposomes still contained 83 per cent and 67 per cent of drug respectively after 24 h at 37 degrees C.

Phenotypic variation of Staphylococcus epidermidis in infection of transvenous endocardial pacemaker electrodes

Coagulase-negative staphylococci isolated from a patient with a pacemaker electrode infection were extensively evaluated by phenotypic and genotypic characterization. Findings from this evaluation were striking because different colony morphologic subtypes were recovered from blood and resected pacemaker electrodes. Staphylococci from each colony subtype (LBL, LBV, LBP, LBS) were identified as slime-producing strains of Staphylococcus epidermidis sensu stricto. Direct plating of isolates from a restricted electrode revealed a mixture of colony phenotypes when examined on a high-salt, low-glucose medium, Memphis agar. Bacteriophage typing employing 17 different phages and plasmid profile analysis were largely unsuccessful in further characterizing bacterial cells of each of the four colony morphotypes. On the other hand, restriction endonuclease analysis by EcoRI digestion of the chromosomal DNA demonstrated the probable common clonal origin of the four colony phenotypes.

Pathogenesis and therapy of Pseudomonas aeruginosa keratitis

Considerable progress has been made recently in understanding the pathogenesis of pseudomonas keratitis including its adhesion, invasion and the role of the glycocalyx. Adhesion to epithelial cells has been shown in vitro to depend on pili but their relationship to the glycocalyx biofilm has yet to be explored. The actions of its toxins and proteases have been established by studying the effects of deletion mutants. However, in animal models pseudomonads have been unable to cause an invasive infection in the presence of an intact corneal epithelium and have required associated trauma. Why early necrosis of the cornea can occur in an immunocompetent individual is still not clear but may reflect the delayed access of polymorphonuclear cells (PMNs) in an avascular tissue. Whether the corneal necrosis that later occurs is then due to the PMNs or pseudomonas toxins is not established. Topical fortified gentamicin therapy remains the treatment of choice for early infection. The use of medical and surgical adjunctive therapy is often needed to cope with the complications of advanced disease and has been reviewed.

Bactericidal interactions of a beta-lactam and beta-lactamase inhibitors in experimental Pseudomonas aeruginosa endocarditis caused by a constitutive overproducer of type Id beta-lactamase

We investigated the in vitro and in vivo effects of a combination of a beta-lactam (ceftazidime) and a beta-lactamase inhibitor (dicloxacillin) to synergistically kill a ceftazidime-resistant variant, Pseudomonas aeruginosa PA-48, which overproduces type Id cephalosporinase constitutively. In vitro, dicloxacillin plus ceftazidime exerted bactericidal synergy at approximately 10(5) CFU/ml of inoculum (but not at approximately 10(7)-CFU inoculum), whereas other beta-lactamase inhibitors (sulbactam, clavulanic acid) showed no enhanced killing of PA-48 when combined with ceftazidime at clinically achievable levels for each agent. Dicloxacillin was a potent competitive inhibitor of the extracted Id cephalosporinase from strain PA-48 in short-term comixture studies (less than 10 min [Ki = 2 nM]); in contrast, longer-term comixture studies (90 min) indicated that dicloxacillin functions as a competitive substrate for the enzyme. Growth of PA-48 cells in the presence of dicloxacillin (12.5 to 100 micrograms/ml) had no significant effect on the production rates or functional activity of the Id enzyme. In experimental aortic valve endocarditis due to the ceftazidime-resistant variant (PA-48), rabbits received either no therapy, ceftazidime (25 mg/kg intramuscularly, every 4 h), or ceftazidime plus dicloxacillin (200 mg/kg intramuscularly, every 4 h). The combination regimen reduced mean bacterial densities of PA-48 within cardiac vegetations significantly below those in the other groups at both days 3 and 6 of treatment (P less than 0.005). However, mean vegetation bacterial densities remained greater than 6 log10 CFU/g in the combined treatment group. This modest in vivo synergistic effect (as compared to striking in vitro synergy at approximately 10(5)-CFU inoculum) most likely reflects the high densities of PA-48 achieved in vivo within cardiac vegetations (greater than 8 log10 CFU/g).