Differential response to benzylpenicillin in vivo of tolerant and non-tolerant variants of Streptococcus sanguis II.

A variant that was highly tolerant to benzylpenicillin was obtained from a non-tolerant clinical isolate of Streptococcus sanguis II by repeated exposure to penicillin. The rabbit model of endocarditis was used to investigate the efficacy of a high dose regimen of benzylpenicillin (250 mg/kg; peak serum concentration c. 25 mg/l) in the prophylaxis and treatment of endocarditis during challenge or infection with the non-tolerant parent strain or its tolerant variant. The two strains exhibited a similar capacity to initiate infection. A single dose of penicillin administered 0.5 h before bacterial challenge protected six of nine rabbits infected with the non-tolerant parent strain, but none of nine infected with the tolerant variant. Treatment of established infection with penicillin administered twice daily for four days cured eight of 13 (61%) rabbits infected with the non-tolerant parent strain, but only one of 14 (7%) rabbits infected with the tolerant variant. These results support the view that tolerance to penicillin has therapeutic implications.

Comparison of fast (one-step) and interrupted slow cooling methods using a range of intracellular and extracellular cryoprotectants for the freeze-preservation of Plasmodium yoelii-infected mouse erythrocytes.

Several cryoprotectants were compared using two different cooling procedures for the cryopreservation of both normal and Plasmodium yoelii-infected mouse erythrocytes. Fast cooling to -196 degrees C by direct plunge into liquid nitrogen followed by rapid thawing in a 37 degree C water bath protected uninfected and parasitized erythrocytes against freeze-thaw damage significantly better than an interrupted slow cooling procedure in which cells were frozen to -70 degrees C at a rate of -1 degree C/min followed by storage in liquid nitrogen. Using the former procedure, the highest percentage erythrocyte recoveries (over 85%) and shortest pre-2% parasitaemia times (equivalent to unfrozen cells) in mice challenged with thawed parasitized blood were observed with the intracellular (penetrating) cryoprotectants glycerol or dimethylsulphoxide (Me2SO). At the concentrations used in this study, the extracellular (non-penetrating) cryoprotectants, hydroxyethylstarch, dextran or polyvinyl pyrrolidone were significantly less effective at protecting against freeze-thaw damage. Some evidence of freeze-thaw damage was obtained in cells fast frozen at low cell density (less than or equal to 10(4)/ml) in glycerol or Me2SO. This effect was masked when higher cell densities (10(7) to 10(8)/ml) were used. Addition of the anti-oxidant and membrane stabilizing molecules, vitamin E, sodium selenite or selenomethionine to infected erythrocytes just before and during cryopreservation, did not improve, and in one case inhibited, subsequent development in challenged mice.

Comparison of ceftazidime, cefuroxime and methicillin in the treatment of Staphylococcus aureus endocarditis in rabbits.

Ceftazidime, cefuroxime and methicillin proved equally effective in the therapy of experimental Staphylococcus aureus endocarditis in rabbits with a dosing regimen of 40 mg/kg intramuscularly at 8-hourly intervals for three days. Treated animals all demonstrated a thousand to 10,000-fold reduction in the levels of bacteria in the vegetations compared with untreated controls. In-vitro sensitivities of the organism to the test antibiotics were not predictive of therapeutic efficacy in vivo.

Evaluation of a range of antimicrobial agents against the parasitic protozoa, Plasmodium falciparum, Babesia rodhaini and Theileria parva in vitro.

Eighteen antimicrobials commonly used in tissue culture were screened in three different protozoan test systems in order to establish their suitability for routine inclusion in protozoal cultivation systems. The human malaria parasite, Plasmodium falciparum, was inhibited by more than half the antibiotics tested at concentrations recommended for normal tissue culture use. Eight compounds were well tolerated and thus could be used prophylactically to prevent microbial contamination. These antimicrobials were the bactericidal aminoglycoside antibiotics, streptomycin, gentamicin and kanamycin, the bacteriostatic protein synthesis inhibitors, chloramphenicol and chlortetracycline and the antifungals, 5-fluorocytosine, nystatin and amphotericin B. Babesia rodhaini and Theileria parva were less sensitive than P. falciparum and tolerated all 18 compounds at concentrations well above 100 micrograms ml-1. Extension of the study to examine direct antiprotozoal action of these and other antimicrobials not normally used in culture confirmed that P. falciparum was significantly more sensitive than the other parasites. Tylosin, rifamycin, gramicidin D and valinomycin were all strongly antimalarial with IC50 values of 0.245, 1.20, 1.3 X 10(-3) and 1.9 X 10(-3) micrograms ml-1 respectively. This compares with a value of 1.35 X 10(-2) micrograms ml-1 for the standard antimalarial, chloroquine. Only valinomycin and, more particularly, gramicidin D were significantly active against B. rodhaini and T. parva. Gramicidin D was more effective, but more toxic, than the standard antiprotozoal agents tested at curing in vivo malarial and babesial infections in mice.

Evaluation of ceftazidime, ampicillin and chloramphenicol in experimental Haemophilus influenzae type b meningitis.

In an infant rat model of Haemophilus influenzae, type b meningitis, where treatment was given 24 and 48 h after infection, the dose of ceftazidime required to eradicate the infection from the CSF of half the animals (CD50) ranged from less than 0.15-1.5 mg/kg/dose. The accompanying blood infections were marginally less responsive to therapy with CD50 values ranging from 0.5-3.9 mg/kg/dose. Comparable data for ampicillin were 12.5-40 mg/kg/dose and 20- greater than 200 mg/kg/dose for the CSF and blood infections while those for chloramphenicol were 18- greater than 100 mg/kg/dose and 22- greater than 100 mg/kg/dose for the CSF and blood infections respectively. Investigation of the relative rates of kill in vivo showed that all three drugs rapidly reduced the bacterial numbers to minimal levels. However, whereas ceftazidime completely eradicated the infection, chloramphenicol, and to a lesser extent, ampicillin-treated rats experienced substantial relapsing. Ceftazidime penetrated into the CSF of infected and uninfected rats slightly better than ampicillin--7.3% compared to 4.0% of the corresponding blood levels respectively. These results indicate that ceftazidime is significantly more active in the infant rat model of H. influenzae, type b meningitis than ampicillin or chloramphenicol.

A comparison of saponin with other adjuvants for the potentiation of protective immunity by a killed Plasmodium yoelii vaccine in the mouse.

The protective immunity conferred by subcutaneous injection of outbred CD-1 mice with a killed Plasmodium yoelii (YM strain) vaccine was strongly potentiated by saponin. By adjusting the dose of antigen, the number of immunizations and the number of living parasites in the challenge infection, conditions were defined where antigen alone was non-protective but 100% protection was obtained by the addition of saponin. Inbred BALB/c, CBA/CA and C57 B1 mice were much less responsive than the CD-1 mice. The following adjuvants were compared with saponin: mineral oil emulsions (Freund's incomplete and complete adjuvants); A1(OH)3(Alhydrogel); bacteria and synthetic bacterial derivatives (Bordetella pertussis, Corynebacterium parvum and muramyl dipeptide); surface active materials (digitonin, vitamin A, Arquad 18, dimethyldioctadecyl ammonium bromide, and the polyene antibiotics, Nystatin and Amphotericin B). None of these adjuvants were as effective as saponin, although FCA, A1(OH)3 and C. parvum augmented immunity considerably. The possible reasons for the efficacy of saponin as an adjuvant for protozoal vaccines are discussed. The P. yoelli/mouse system provides a sensitive and rapid screening assay for comparison of potential adjuvants suitable for use with a malaria vaccine.