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PubMed1980

Selection of variants of Pseudomonas aeruginosa resistant of Beta-lactam antibiotics.

In broth cultures of Pseudomonas aeruginosa, containing carbenicillin or azlocillin, regrowth occurred after a period of bactericidal action, to reach visible proportions overnight. Regrowth in the presence of relatively high concentrations of carbenicillin or azlocillin could not be accounted for on the basis of growth of resistant variants nor as a result of drug inactivation. On the other hand, resistant variants could be selected from the regrowth which occurred at concentrations of carbenicillin or azlocillin only slight in excess of the minimum inhibitory concentrations (MIC). Antibiotic resistant variants could also be isolated from individual colonies growing on agar plates containing carbenicillin, ticarcillin, azlocillin or piperaccillin at concentrations above the MIC for the majority of the population. Two types of resistant variant were isolated. The first showed a 2-5 fold increase in resistance to carbenicillin, ticarcillin, azlocillin and piperacillin while Beta-lactamase production in these variants appeared to be unchanged. The second type of resistant variant showed unchanged sensitivity to carbenicillin and ticarcillin, or only a slight increase in resistance, whereas resistance to azlocillin and piperacillin was increased as much as 40-fold or more. These variants showed increased constitutive Beta-lactamase production and may be derepressed mutants of the parent culture. Variants of this type were readily selected by culture in the presence of azlocillin or piperacillin but only infrequently as a result of culture in the presence of carbenicillin or ticarcillin. The existence in cultures of P. aeruginosa of variants showing elevated Beta-lactamase production may account at least in part for the effect of inoculum size on the activity of azlocillin and piperacillin against P. aeruginosa and the marked discrepancy between MIC and minimum bactericidal concentration (MBC) which is characteristic of the ureido penicillins.

Animals↗
PubMed1987

Evaluation of ciprofloxacin's synergism with other agents by multiple in vitro methods.

The efficacies of ciprofloxacin, ceftizoxime, azlocillin, mezlocillin, and amikacin (minimal inhibitory concentration and minimal bactericidal concentration) against six Pseudomonas aeruginosa, six Enterobacteriaceae, and six group D streptococcal strains were evaluated using both agar and broth susceptibility methods, two inoculum sizes (5.7 log10 colony-forming units (cfu)/ml and 7.7 log10 cfu/ml), and aerobic and anaerobic incubation conditions. The results showed agreement between broth and agar methods of susceptibility determination; inoculum effects with beta-lactam antimicrobials; and decreased susceptibility to amikacin under anaerobiasis. Ciprofloxacin combined with azlocillin, ceftizoxime, or aminoglycosides in broth microdilution checkerboards against 100 gram-negative bacilli and gram-positive cocci demonstrated that ciprofloxacin combined with azlocillin or ceftizoxime was synergistic against at least 50 percent of P. aeruginosa and Serratia marcescens isolates and that ciprofloxacin combined with amikacin was synergistic against at least 50 percent of S. marcescens and Staphylococcus aureus isolates. Ciprofloxacin and azlocillin in combination were evaluated by microdilution checkerboard, agar dilution, and broth macrodilution time-kill methods at two inoculum sizes to assess antibacterial activity. Comparison between in vitro combination methods showed the following: the presence or absence of checkerboard synergism (as defined by the fractional inhibitory concentration index and the fractional bactericidal concentration index) with ciprofloxacin and azlocillin did not correlate with time-kill results; and good agreement between methods when comparing broth macrodilution time-kill (3 log10 cfu/ml or more decrease) with antimicrobial combinations at a single concentration in both agar and microdilution broth for ciprofloxacin and azlocillin. Rabbit studies using subcutaneous dialysis membrane chambers inoculated with six P. aeruginosa, six Enterobacteriaceae, and six group D streptococcal strains were performed using ciprofloxacin, azlocillin, ceftizoxime, and amikacin alone and in combination as therapy. In vitro testing of antibiotic combinations that provided the best prediction of in vivo outcome were combination antibacterial activity (3 log10 cfu/ml or more decrease) at 24 hours using either broth macrodilution time-kill or antimicrobial combinations at a single concentration in either agar or broth (microdilution). For the most efficacious in vivo combination, ciprofloxacin plus azlocillin, there was in vitro correlation with in vivo outcome for 17 of 18 isolates.

Animals↗
PubMed1986

In vitro models for the study of combination antibiotic therapy in neutropenic patients.

Neutropenic patients are at risk of serious infection caused by gram-negative bacilli and staphylococci. The mortality rate associated with gram-negative bacteremia in these patients is extremely high, especially in those with persistent and profound granulocytopenia. In these latter patients, the best results have been obtained by administering combinations of antibiotics in which both agents are active and/or show in vitro synergism against the infecting organism. Most combinations include an aminoglycoside such as amikacin and a broad-spectrum beta-lactam antibiotic, such as azlocillin, mezlocillin, piperacillin, or ceftazidime. The International Antimicrobial Therapy Project Group of the European Organization for Research and Treatment of Cancer has completed several studies evaluating various antibiotic combinations in the empiric treatment of febrile neutropenic patients. These trials have evaluated cephalothin plus gentamicin, carbenicillin plus gentamicin, and cephalothin plus carbenicillin; carbenicillin plus amikacin and carbenicillin plus amikacin plus cefazolin; azlocillin plus amikacin, ticarcillin plus amikacin, and cefotaxime plus amikacin; and azlocillin plus amikacin versus ceftazidime plus long- or short-course amikacin. The preclinical evaluation of antibiotic combinations usually involves the in vitro testing of antibiotics alone and in combination by the checkerboard method or with the use of time-kill curves. However, these methods expose the bacterial culture to a static or constant concentration of the drugs. During the in vivo treatment of infections, bacteria are exposed to changing concentrations of antibiotics, which are contingent on the individual pharmacokinetics of these drugs. We have designed a two-compartment in vitro pharmacokinetic model that allows the simultaneous study of the activity of two antibiotics with similar or different half-lives against a number of bacteria. Amikacin and azlocillin have been studied alone and in combination in this model against Pseudomonas aeruginosa, a frequent cause of bacteremia in neutropenic patients. In pharmacologically relevant doses, amikacin alone produced rapid bacterial killing, followed by regrowth of resistant subpopulations. Azlocillin alone produced a more gradual reduction of the bacterial inoculum, with ultimate bacteriostasis. Amikacin plus azlocillin produced rapid and complete eradication of the organism. In vitro pharmacokinetic models may prove to be more predictive of clinical outcome than are traditional static in vitro methods used to study antibiotic combinations.

Agranulocytosis↗
PubMed1991

Single daily dosing of amikacin in an in-vitro model.

The pharmacodynamics of amikacin given as a single daily dose was compared with standard divided dosing in an in-vitro model of infection. This model allows the exposure of log phase bacteria to changing concentrations of antibiotics that simulate the kinetics of the drugs in human patients. Two strains of Pseudomonas aeruginosa, one sensitive and one resistant to azlocillin were studied (MICs for amikacin were 16 and 8 mg/l respectively). Simulated drug regimens included: amikacin 400 mg q 8 h; amikacin 1.2 g q 24 h; and azlocillin 4 g q 12 h. Each regimen alone and both combinations of amikacin plus azlocillin were studied. With both amikacin regimens initial rapid killing was followed by regrowth of resistant subpopulations. Azlocillin alone produced minimal killing of the resistant strain and moderate killing with ultimate bacteriostasis of the susceptible strain. Bacterial regrowth was prevented with both combination regimens with the single daily dose of amikacin plus azlocillin producing the most rapid and complete killing, especially of the azlocillin resistant strain. These data support further clinical studies of single daily dosing of aminoglycosides.

Amikacin↗
PubMed1979

[Current chemotherapy of infections caused by Pseudomonas aeruginosa (author's transl)].

In addition to carbenicillin, the newer beta-lactam antibiotics such as ticarcillin and azlocillin are now available for the chemotherapy of Pseudomonas aeruginosa infections. We investigated the in vitro effect of these antibiotics on 233 isolates from clinical material. We were particularly careful, when choosing the experimental material, to exclude copies of the same individual strain, and we achieved this by combining various epidemiological typing procedures. A comparison of carbenicillin, ticarcillin and azlocillin according to the concentrations at which half of the 233 strains were inhibited showed the ticarcillin values to be higher than those of azlocillin by a factor of 2.1, and carbenicillin values to be higher than those of azlocillin by a factor of 4.9. Individual strains also occurred in which the inhibitory concentration for azlocillin was higher than that of carbenicillin (5 strains) or ticarcillin (11 strains). In 17 out of the 233 isolates no therapeutic success would have been within reach even with the newer beta-lactam antibiotics. The use of ticarcillin and azlocillin permits an extension of the indications for therapy with beta-lactam antibiotics in P. aeruginosa infections, from hitherto 76%, to 90% of the cases. If one includes the aminoglycosides gentamycin, tobramycin, sisomycin and amikacin in the therapeutic armoury, then the proportion of in vitro sensitive strains of P. aeruginosa in the material submitted for examination rises to 98%.

Aminoglycosides↗
PubMed1982

[In vitro effect of acylureido penicillins and mezlocillin on anaerobic sporeless gram-negative rods of genus Bacteroides].

313 anaerobic gram-negative rods of the Bacteroides genus isolated from 295 clinical specimens were tested for their sensitivity to azlocillin and mezlocillin using the dilution method with agar plates. 25 mg/l of azlocillin inhibited 80.2% of the strains and the same concentration of mezlocillin inhibited 86.9%. The somewhat higher in vitro activity of mezlocillin is also seen by the lower mean minimal inhibitory concentration, which is approx. 23.6 mg/l for mezlocillin and 27.6 mg/l for azlocillin. The highest resistance for both drugs were found with B. distasonis (14 strains), followed by B. thetaiotaomicron (63 strains), B. fragilis (161 strains) and B. uniformis (7 strains). The good sensitivity of strains belonging to the B. melaninogenicus group corresponds to the well-known sensitivity of these micro-organisms to penicillins. On the basis of a "break point" of 25 mg/l, a decrease in the proportion of sensitive strains was found between 1974 and 1979. 50 mg/l, a concentration which can be maintained over a longer period of time if applied in a suitable manner, will inhibit 97.4% (azlocillin) and 98.4% (mezlocillin) of the strains. Thus in our opinion azlocillin and especially mezlocillin are suitable drugs for the initial therapy of pyogenous infections in which intestinal Bacteroidaceae are of pathogenic significance.

Azlocillin↗