Fundamental Study on the Chemotherapy of Experimental Pseudomonas Infection

Abstract

Pseudomonas aeruginosa is an ubiquitous organism in nature and has been thought to be low-virulent. The discovery and practical use of penicillin followed by exploitation of various antibiotics such as streptomycin and chloramphenicol, lead to the effective treatment of infectious diseases that were due to certain bacterial species. Gram-positive pyogenic cocci, representative of such organism, were successfully eradicated by therapeutic usage of these drugs whereas the cases of infection due to Gram-negative rods known as low-virulent or even non-pathogenic became increasingly important. This phenomenon is understood as “microbisme sèlectioné et substitute” or “opportunistic infection”.
Although the discovery of gentamicin (GM) and penicillin delivatives such as carbenicillin and sulbenicillin resulted in fairly effective control of human P. aeruginosa infection, certain predisposed patients developed fatal infection due to this organism.
In the present study, the author attempted to find an effective way to control human P. aeruginosa infection with a combined administration of GM, one of the most eff ective antibiotics for P. aeruginosa, and several samples of γ globulin (γ-G) in tests on experimental P. aeruginosa infection in mice. The bacterial strain used in this experiment was P. aeruginosa NC-5. Minimum inhibitory concentration (MIC) of GM against P. aeruginosa NC-5, estimated by broth dilution method, was 6.25 mcg/ml.
ICR strain male mice,3 to 4 weeks old (about 15 grams), w ere infected with an intraperitoneal injection of 0.5 ml of bacterial suspension. GM combined with or without -G was intramuscularly injected shortly after the inoculation. The mice were o brserved for survival up to 3 days.
The median lethal doses (LD₅₀) of strain NC-5 to mice, estimated with and without mucin, were ca 10 ^3.2 cells/ml and ca 10 ^6.2 cells/ml, respectively.
The results of therapeutic experiments obtained were as follows:
1. The median effective doses of GM for the mice challenged with 1,000 LD₅₀ and 100LD₅₀ of NC-5 were 312.5 mcg/0.1 ml and 70.7 mcg/0.1 ml, respectively.
2. Neither fractionated γ-G (150 mcg/0.1 ml) nor γ-G in. whole serum (500 mcg/0.1ml) of normal rabbits protected the mice against the challenge of 1,000 LD₅₀ of the organism. But therapeutic effects were observed in the combined administration of 50mcg/0.1 ml (1/6 ED₅₀) of GM and 250 mcg/O.1 ml of serum (or 150 mcg/0.1 ml of γ-G).
3. Anti-NC-5 rabbit serum was more effective than normal serum. That is, therapeutic effects were observed in the group treated with 200 mcg/0.1 ml of the antiserum or its γ-G. The combined usage of 25 mcg/0.1 ml (1/8 ED₅₀) of antiserum γ-G and a 50 mcg/0.1 ml (1/6 ED₅₀) of GM was markedly synergized.
4. The effect, of γ-G combined with GM was verified as the activity of a specific antibody by its absorption test with corresponding antigen.
5. IgG and IgM fractions from rabbit antiserum for NC-5 were compared with each other for their therapeutic effect. IgG, was more effective than IgM despite its lower agglutination titer.
6. In the group of mice treated with GM combined with γ-G (or IgG) the viable bacterial number in peritoneal exsudate was decreased and the animals survived. In the use of IgM instead of IgG, however, the decrease of viable bacterial number was less and the mice failed to survive.
From the above described results, it was concluded that synergism between the antiserum or its γ-G and GM was observed in the treatment of experimental P. aeruginosa infection in mice. The mechanism of this synergism claimed that the bacteria were easily phagocytized by the opsonization with the antiserum or its γ-G, and GM exert bactericidal activity in cooperation with the host cells.