Relationship Between Gentamicin Susceptibility Criteria and Therapeutic Serum Levels for Pseudomonas aeruginosa in Mouse Infection Model

Abstract

In this study estimations of in vivo and in vitro gentamicin susceptibility for a series of strains of Pseudomonas aeruginosa were compared. The series included an extremely susceptible strain, typically susceptible strains by current susceptibility criteria, and strains with enzymatic and permeability-mediated resistance. In vivo testing was done by using a mice protection test involving six 1-h doses of gentamicin and an inoculum of 50 50% lethal doses of P. aeruginosa. Both normal mice and cyclophosphamide-treated mice were used. It was found that peak serum levels and serum levels of gentamicin obtained just prior to the sixth dose (fifth dose trough levels) required for protection were much higher than miniimal inhibitory concentrations (MICs) or minimal bactericidal concentrations (MBCs) obtained in high-cation medium. However, first dose trough levels were similar to MICs or-MBCs. Only an extremely susceptible strain, 280, could be treated at antibiotic dosages and serum levels which are considered likely to be safe in humans. A distinct inoculum effect was found in the mice tests, with a 10-fold increase in inoculum producing a 4-fold increase in the amount of gentamicin required, but no inoculum effect was found for MICs. These results suggest that current susceptibility criteria in use for gentamicin and P. aeruginosa overestimate gentamicin susceptibility, particularly when low-cation growth medium is used for susceptibility testing and when treating disseminated infection.Gentamicin, either alone or in combination with carbenicillin, is currently considered to be one of the most useful antibiotics for treatment of infection by Pseudomonas aeruginosa. Despite many years of clinical experience, there is still considerable doubt about the efficacy of gentamicin in the treatment of Pseudomonas sepsis. Poor survival rates reported in several studies ofPseudomonas bacteremia suggest that gentamicin is not highly effective in disseminated infections (8,14,16), yet most strains of P. aeruginosa are considered susceptible by current laboratory in vitro susceptibility testing results.In the present study we examined the relationship of gentamicin susceptibility to in vivo efficacy of gentamicin in mouse models of disseminated infection by using a series of isogenic P. aeruginosa strains with different R-factors and a nonisogenic series varying in gentamicin susceptibility and virulence. Currently if strains have miniimal inhibitory concentrations (MICs) of -6 they are regarded as susceptible (17). We have included infection of cyclophosphamidetreated mice as a model of infection of hosts with compromised host defenses. Because the apparent susceptibility of P. aeruginosa to aminoglycosides is very highly influenced by media conditions (9, 10, 20,

The inoculum effect and band‐pass bacterial response to periodic antibiotic treatment

“…In the clinical setting, the presence of IE is undesirable as it can cause overestimation of in vitro resistance ( Duraisingh et al, 1999 ), increased mortality rates of infected hosts due to administration of insufficient doses of antibiotics ( Soriano et al, 1990 ), and increases in the generation rate of resistant pathogens ( Martinez and Baquero, 2000 ). IE has been observed both in animal models of infection ( Nicas and Bryan, 1978 ; Mizunaga et al, 2005 ) and in human patients ( Moshkowitz et al, 1995 ; Lai et al, 2003 ), suggesting its clinical relevance. To date, however, there is a general lack of mechanistic understanding of IE.…”

Determinants of the efficacy of tobramycin therapy against isogenic nonmucoid and mucoid derivatives of Pseudomonas aeruginosa PAO1 growing in peritoneal chambers in mice

“…Therefore, at various times after injection, the levels of tobramycin in the blood and in saline-containing chambers implanted in the peritoneum were assessed (Table 1). Con- (16,18). In contrast, in the peritoneal chambers, maximal tobramycin concentrations of only 3.8 pg/ml were obtained after 15 min, but concentrations declined much more slowly (half-life of approximately 5 h) and remained above the MICs for P. aeruginosa for 8 h after the tobramycin injection.…”

“…These chambers are then inserted into the peritoneal cavities of laboratory mice. The bacteria then undergo a classical bacterial growth curve to a peak of 108 to 109 bacteria per ml within 16 h, after which numbers of viable bacteria remain stable for some months. Removal of the chambers and extraction of their contents allow full recovery of the in vivo-grown organisms.…”