In vitro activity of RP 59500 (quinupristin/dalfopristin) against antibiotic-resistant strains of Streptococcus pneumoniae and enterococci.

The activity of RP 59500 (quinupristin/dalfopristin) was evaluated in vitro against antibiotic-resistant strains of Streptococcus pneumoniae (N = 15) and Enterococcus spp. (N = 43). By broth dilution MIC tests RP 59500 was highly active against penicillin-resistant S. pneumoniae and vancomycin-resistant Enterococcus faecium, but showed poor activity against E. faecalis. In time-kill studies the drug was rapidly bactericidal against S. pneumoniae but failed to kill most enterococci, even in the presence of gentamicin or human serum.

Effect of insulin treatment on the susceptibility of the diabetic rat to Escherichia coli-induced pyelonephritis.

Escherichia coli-induced pyelonephritis was studied in untreated alloxan-diabetic rats, insulin-treated diabetic rats, glucose water-drinking (diuresing) nondiabetic rats, and tap water-drinking (nondiuresing) nondiabetic rats following injection of E. coli either into the emptied urinary bladder, into the left kidney, or intravenously. For prevention of an ascending infection in the right kidney, the right ureter was ligated and transected immediately prior to bladder or intrarenal inoculation. These experiments established that in normal rats ascending renal infection alone occurred following introduction of small inocula into the bladder--and then only when facilitated by diuresis. In diabetic rats both ascending and hematogenous renal infection occurred following introduction of small inocula into the bladder. Insulin treatment that reduced hyperglycemia also reduced glycosuria and restored urinary antibacterial activity against small inocula of E. coli but only partially reduced polyuria and prevented hematogenous but not ascending infection. Thus, hyperglycemia was probably the major factor promoting hematogenous renal infection, whereas polyuria--and therefore vesicoureteral reflux--was the major factor promoting ascending infection.

Susceptibility to experimental Candida albicans urinary tract infection in the rat.

The urinary tract is resistant to ascending infection due to Candida albicans. Host and microbial factors that may alter such resistance were evaluated in the rat after inoculating C. albicans into the urinary tract. Diuresis, diabetes, candidal germ-tube formation, and vaginal Candida colonization (alone or in concert) failed to promote ascending urinary tract infection with two vaginal isolates of C. albicans capable of producing renal infection by the hematogenous route. Both germinated and ungerminated Candida adhered poorly to bladder mucosa. Unlike prior urinary tract infection due to an enterococcus that failed to agglutinate Candida in vitro, prior urinary tract infection with a mannose-binding strain of Escherichia coli that agglutinated C. albicans in vitro enhanced adherence of C. albicans to bladder mucosa and increased susceptibility to ascending C. albicans urinary tract infection. Thus, resistance to Candida ascending urinary tract infection is likely to be due to poor adherence of C. albicans to normal bladder mucosa.

Oral temafloxacin versus vancomycin for therapy of experimental endocarditis caused by methicillin-resistant Staphylococcus aureus.

We compared oral temafloxacin, a new fluoroquinolone agent, with vancomycin, each with and without rifampin, in the therapy of rats with aortic valve endocarditis caused by a clinical isolate of methicillin-resistant Staphylococcus aureus. The temafloxacin, vancomycin, and rifampin MICs and MBCs were 0.78 and 1.56, 1.56 and 3.13, and less than 0.024 and 0.78 microgram/ml, respectively. The animals were classified into the following six treatment groups: vancomycin (60 mg/kg) +/- rifampin (6 mg/kg) each intramuscularly every 12 h for 5 days; temafloxacin (100 mg/kg) orally +/- rifampin (6 mg/kg) intramuscularly every 12 h for 5 days; rifampin (6 mg/kg) intramuscularly every 12 h for 5 days; and untreated controls. All regimens with either vancomycin or temafloxacin resulted in improved survival over controls, but only temafloxacin regimens resulted in a significant reduction in bacterial counts in vegetations. These data support further investigation of the efficacy of temafloxacin in treating serious infections caused by methicillin-resistant S. aureus.

Absence of a postantibiotic effect in experimental Pseudomonas endocarditis treated with imipenem, with or without gentamicin.

We found an in vitro postantibiotic effect (PAE) of 3-4 h for imipenem and of approximately 5 h for imipenem plus gentamicin against Pseudomonas aeruginosa. We therefore evaluated these antibiotics in a rat model of pseudomonas endocarditis. A rapid bactericidal effect was initially observed in vegetations from rats treated with imipenem alone or in combination with gentamicin. Bacterial counts rose rapidly, however, as soon as levels of imipenem in vegetations fell below the minimal inhibitory concentration (i.e., no PAE was demonstrated). Levels of gentamicin in vegetations were similar to those that had enhanced the bactericidal effect of imipenem and had resulted in a 5-h PAE in vitro. The presence in vitro of a PAE for imipenem, with or without gentamicin, does not necessarily predict its presence in vivo in pseudomonas endocarditis in the rat.

Postantibiotic effect of penicillin plus gentamicin versus Enterococcus faecalis in vitro and in vivo.

A persistent suppression of bacterial growth following a brief exposure to an antibiotic (postantibiotic effect [( PAE]) has been described for a variety of antibiotics and microorganisms. If a PAE is present in vivo, antibiotic levels in tissue at the site of infection may decrease below the MIC without bacterial regrowth in the latter portion of the dosing interval. In the present studies, a PAE was sought in vitro and in vivo for penicillin G plus gentamicin versus Enterococcus faecalis. The studies demonstrated that increasing concentrations of gentamicin caused an increased rate of bactericidal action and an increasingly prolonged PAE in vitro. The combination of penicillin and gentamicin, in addition to more rapid killing, exhibited a more prolonged PAE than did penicillin alone. However, unlike these in vitro findings, no PAE could be demonstrated in vivo in rats with experimental left-sided enterococcal endocarditis treated with penicillin plus gentamicin. This suggests that antibiotic vegetation levels should be maintained above the MIC throughout the dosing interval to prevent loss of efficacy as a result of bacterial regrowth.

Free water reabsorption during saline diuresis in experimental enterococcal pyelonephritis in rats.

Inability to attain Umax after overnight dehydration is the earliest functional abnormality in human and experimental pyelonephritis caused by diverse microorganisms. In order to characterize the defect in Umax in experimental enterococcal pyelonephritis, another index of renal concentrating ability. TcH2O, was determined during saline loading. Normal TcH2O depends on adequate sodium chloride delivery and reabsorption in the ascending limb of Henle's loop and water reabsorption from the collecting duct. Rats with early pyelonephritis, 3 days after intravenous injection of enterococci, were compared with normal rats in studies of Umax during hydropenia and TcH2O during 1.2% saline infusion. Mean Umax in infected rats was significantly lower than in uninfected rats (1120 vs. 2767 mOsm/kg H2O) (p less than 0.01), but CIn in infected rats was not significantly different from that in uninfected rats (0.96 vs. 0.89 ml/min per 100 gm) (p less than 0.05). During saline diuresis, maximal Cosm/CIn was more than 35% in both the normal and infected rats. The relationship between TcH2O/CIn and Cosm/CIn was linear in both groups, and the r, slope, any y intercept of the regression equation of TcH2O/CIn vs. Cosm/CIn in infected rats were not significantly different from those in normal rats. During saline diuresis the regression of sodium excretion UNaV/CIn) vs. Cosm in infected rats was not significantly different from that in control rats. The finding of normal TcH2O during saline loading suggests that reabsorption of increasing amounts of sodium chloride from the ascending limb of Henle's loop and reabsorption of water in the collecting duct are normal in early pyelonephritis.

The importance of pharmacodynamics in determining the dosing interval in therapy for experimental pseudomonas endocarditis in the rat.

The efficacy of ciprofloxacin, BMY-28142, and ceftazidime was compared in vitro and in experimental left-sided endocarditis due to Pseudomonas aeruginosa in the rat. The dose, dosing interval, and duration of therapy were varied, and the resulting antibiotic levels in serum and vegetations were correlated with bacterial clearance from vegetations. These studies demonstrated that beta-lactams such as BMY exhibited a slow rate of bactericidal action and had no postantibiotic effect against P. aeruginosa in vitro or in vivo. As a consequence, BMY had to be given in multiple doses at relatively short intervals during which concentrations of antibiotics in vegetations were continuously in excess of the MBC for the pathogen. The earlier onset of rapid bactericidal action and the prolonged postantibiotic effect of ciprofloxacin (demonstrated in vivo and in vitro) were, in all likelihood, the factors that allowed the successful use of fewer doses of this antimicrobial agent at relatively longer dosing intervals.

In vitro postantibiotic effect of daptomycin (LY146032) against Enterococcus faecalis and methicillin-susceptible and methicillin-resistant Staphylococcus aureus strains.

The suppression of bacterial growth that persists after brief exposure to antimicrobial agents has been termed the postantibiotic effect (PAE). This pharmacodynamic interaction varies for each microorganism-antimicrobial agent combination. Daptomycin (LY146032) is a new lipopeptide antibiotic with activity against gram-positive organisms. We studied the in vitro bactericidal activities and PAEs of the following drugs: daptomycin compared with penicillin G and vancomycin, without and with gentamicin against Enterococcus faecalis strains; daptomycin compared with nafcillin and vancomycin against methicillin-susceptible Staphylococcus aureus strains; and daptomycin compared with vancomycin against methicillin-resistant S. aureus strains. Daptomycin, alone and when used in combination with gentamicin, exhibited greater bactericidal activity and in general produced a longer PAE than standard effective regimens against the organism strains studied.

The bactericidal activity of magainins against Pseudomonas aeruginosa and Enterococcus faecium.

The antimicrobial activities of two synthetic magainins, MSI 94 and MSI 93, were investigated in vitro against 20 clinical isolates each of Pseudomonas aeruginosa and Enterococcus faecium. The ranges of the MICs of both agents were 6.25-50 mg/L for P. aeruginosa and 3.13-12.5 mg/L for E. faecium. In time-kill studies, the magainins demonstrated rapid, concentration-dependent bactericidal activity against selected isolates of both species. This bactericidal effect was inoculum-dependent for P. aeruginosa in both the logarithmic and stationary phases of growth. Bacterial regrowth was regularly observed after 24 h of incubation in the time-kill studies; this was due to loss of antimicrobial activity during overnight incubation rather than to the development of resistance. More rapid and sustained bactericidal activity was noted when the magainins were combined with either silver nitrate (AgNO3) or gentamicin against P. aeruginosa and with gentamicin against E. faecium. Neither agent produced a post-antibiotic effect on P. aeruginosa.

Activity of cefepime against ceftazidime-resistant gram-negative bacilli using low and high inocula.

Cefepime is a broad-spectrum cephalosporin that is reported to have enhanced activity against ceftazidime-resistant Gram-negative bacilli. In this study the effects of varying inoculum size on in-vitro susceptibility to cefepime and other selected antimicrobial agents were determined by agar dilution MICs and in time-kill studies. Among strains of Pseudomonas aeruginosa (n = 55) and Enterobacter spp (n = 56) that had previously been identified as ceftazidime-resistant, 73% and 96% were susceptible to cefepime (MIC < or = 16 mg/L), respectively, when tested with an inoculum of 10(4) cfu. However, with an inoculum of 10(7) cfu, 98% and 100% of strains were resistant, respectively. Furthermore, the bactericidal activity of cefepime against ceftazidime-resistant isolates was also inoculum-dependent. In time-kill studies, bactericidal action was obtained only at the lowest concentration of organisms (10(4) cfu/mL). beta-Lactamase extracted from an isolate of P. aeruginosa that demonstrated an inoculum effect had a lower affinity for cefepime than for ceftazidime. Overall, cefepime proved to be more resistant to hydrolysis by the beta-lactamase. However, differences in kinetics of the beta-lactamase against cefepime or ceftazidime do not appear to be of consequence in determining susceptibility of P. aeruginosa and Enterobacter spp. at high bacterial densities, since most strains with chromosomally-mediated beta-lactamase are highly resistant.

Gastrointestinal tract colonization with vancomycin-resistant Enterococcus faecium in an animal model.

Vancomycin-resistant enterococci have become important nosocomial pathogens in many institutions. The gastrointestinal tract of susceptible hosts serves as the likely reservoir from which the organism is disseminated. To study factors promoting colonization and the efficacy of decontamination therapy with antimicrobial agents, a model of gastrointestinal colonization with vancomycin-resistant Enterococcus faecium was developed in CF1 mice. At baseline, all animals were colonized with non-vancomycin-resistant enterococci (5.0 log10 CFU/g), but vancomycin-resistant organisms were not detectable. Following gastric inoculation with 5 x 10(8) CFU of a clinical isolate of vancomycin-resistant E. faecium, the strain transiently colonized the gastrointestinal tract of 100% of mice but was undetectable by Day 14 (< or = 2.7 log10 mean CFU/g). In animals who received 5 mg of streptomycin per ml or 250 micrograms of vancomycin per ml in drinking water, colonization with the organism occurred at significantly higher bacterial counts than in controls at 7 days following inoculation (9.4 for vancomycin, 9.2 for streptomycin, and 5.1 log10 mean CFU/g for controls; P < 0.05). Fecal concentrations of vancomycin-resistant E. faecium persisted at high counts through Day 22 in mice receiving these antibiotics, but low counts were also still detected in 3 of 10 control animals. In mice with previously established vancomycin-resistant E. faecium colonization, oral administration of ramoplanin, a lipoglycodepsipeptide to which the strain was susceptible, suppressed growth of all enterococci in feces, including the vancomycin-resistant strain after 7 days of therapy (< or = 3.1 and < or = 3.3 log10 mean CFU/g for vancomycin and streptomycin groups, respectively). All mice had a recurrence of colonization with vancomycin-resistant E. faecium after the ramoplanin was discontinued. In summary, this animal model demonstrates the importance of antibiotics in predisposing to gastrointestinal colonization with vancomycin-resistant Enterococcus spp. Although treatment with ramoplanin temporarily suppressed the organism, recurrence of colonization due to relapse or reinfection occurred.

beta-Lactamase production in experimental endocarditis due to aminoglycoside-resistant Streptococcus faecalis.

We used a beta-lactamase-producing (beta L+) strain of Streptococcus faecalis that also had high levels of resistance to all aminoglycosides to induce experimental endocarditis in rats. The rats were treated for five or 10 days with procaine penicillin, vancomycin, gentamicin, rifampin, or ciprofloxacin (alone or in various combinations), or with penicillin plus clavulanic acid. The levels of penicillin in serum and vegetations declined rapidly in the beta L+-infected rats treated with procaine penicillin alone, unlike the sustained levels of penicillin in either beta L- -infected rats treated with procaine penicillin or beta L+-infected rats treated with penicillin plus clavulanic acid. For the beta L+-infected rats, the enterococcal counts in vegetations were significantly reduced (greater than 3 log10 cfu/g) only by vancomycin and by penicillin plus clavulanic acid. The efficacy of the latter regimen probably resulted from the inhibition of penicillin inactivation by clavulanic acid in vegetations infected with the beta L+ strain. Our in vivo findings document the biologic significance of beta-lactamase production.

Antibiotic treatment of experimental endocarditis due to vancomycin- and ampicillin-resistant Enterococcus faecium.

We compared ciprofloxacin, rifampin, and gentamicin treatments, alone and in combination, for 5 days in the therapy of experimental aortic valve endocarditis in rats caused by a clinical isolate of vancomycin-resistant Enterococcus faecium. The MICs and MBCs of vancomycin, ciprofloxacin, rifampin, and gentamicin were 250 and > 1,000, 3.1 and 6.3, 0.098 and 1.6, and 12.5 and > 50 micrograms/ml, respectively. Infected rats were sacrificed after completing 5 days of therapy. Additional rats within each treatment group were followed for 5 days beyond the last dose of antibiotic therapy. Although survivals in the different groups were not significantly different after 5 days of therapy, survival was significantly better 5 days beyond the last dose of antibiotic therapy in rats treated with rifampin-containing regimens. The combination of ciprofloxacin and gentamicin was bactericidal in vitro and in vegetations from rats with enterococcal endocarditis. Rifampin alone was similarly bactericidal in vivo, but it was not significantly better than rifampin in combination with other antibiotics. Subpopulations resistant to rifampin, but not ciprofloxacin, were detected in the inoculum and in most vegetations during therapy. However, the combination of ciprofloxacin plus both gentamicin and rifampin reduced both the rifampin-susceptible and -resistant population in vegetations of 9 of 10 animals below the level of detection after 5 days of therapy. Nevertheless, a residual enterococcal population apparently remained in numbers of < 2 log10 CFU/g after 5 days of therapy, which resulted in relapse. Perhaps a longer course of therapy would have eliminated this residual population and improved efficacy.