Host determinants of response to antimicrobial agents (concluded)

In vitro susceptibility testing versus in vivo effectiveness

The clinical relevance of susceptibility testing has always been questioned because of the difficulty of correlating in vitro susceptibility testing with in vivo clinical effectiveness. Clearly there have always been host/pathogen factors that influence the clinical outcome that cannot be predicted by the results of susceptibility testing. However, improved understanding of pharmacodynamic and pharmacokinetic parameters has greatly improved the use of antimicrobial agents. Most importantly, the integration of these pharmacodynamic and pharmacokinetic indices has greatly improved the correlation between in vitro susceptibility testing and in vivo clinical effectiveness and allows more realistic breakpoints. Finally, the clinical microbiology laboratory has advanced with improved methods as well as the adaptation of breakpoints that are more realistic.

The immunomodulatory effects of antibiotics: in vitro and ex vivo investigations of 21 substances by means of the lymphocyte transformation test

Besides their antimicrobial activity antibiotics can modulate immune response. The paper provides original data about in vitro and in vivo influence of antibiotics on lymphocyte transformation test (LTT) and gives a comprehensive overview of literature data. In the study presented here the influence of several antimicrobial substances on unstimulated and PHA-stimulated lymphocyte transformation was investigated. The proliferative response was measured as (3H) thymidine uptake by lymphocytes. For initial screening the lymphocyte transformation test was performed on murine lymphocytes in vitro. As a whole 21 antimicrobial substances were tested including representative substances of the most important main groups. As a second step experiments were done with selected substances on human lymphocytes that had shown a distinct influence on murine cells in vitro. At therapeutic concentrations a pronounced stimulation of murine lymphocyte transformation was caused by carbapenems, aminothiazole cephalosporins and imidazoles. Purine analogs had only suppressive effects. However, the increased (3H) thymidine uptake in murine cells could not be regularly reproduced in human lymphocytes and in ex vivo experiments.

Cefdinir-induced modification of the susceptibility of bacteria to the antibacterial activity of human serum and polymorphonuclear neutrophils

The effect of serum on the bactericidal activity of cefdinir, and the ability of the antibiotic to modify the interaction of bacteria with human polymorphonuclear neutrophils were assessed. In the presence of antibiotic, serum-resistant Escherichia coli were sensitised to the bactericidal activity of normal human serum. Cefdinir enhanced opsonophagocytic killing of Escherichia coli and Staphylococcus aureus at suprainhibitory concentrations. Significant potentiation of killing occurred with the combination of inhibitory concentrations of cefdinir, neutrophils and sub-optimal levels of serum opsonins. Pre-exposure of Escherichia coli, but not Staphylococcus aureus, to cefdinir enhanced phagocytic uptake and killing of the antibiotic-damaged bacteria. These results indicate that cefdinir-mediated phenotypic modification of Escherichia coli renders the bacteria susceptible to serum antibacterial activity and phagocytic uptake and intracellular killing.

In vitro killing of erythromycin-exposed group A streptococci by polymorphonuclear leukocytes

After exposure to erythromycin, group A streptococci were tested for susceptibility to the antimicrobial activity of human peripheral blood neutrophils in the absence of the antibiotic. Bacterial susceptibility to phagocytic killing increased after prior exposure to supra-inhibitory levels of erythromycin for even as brief as three minutes. Extended exposure and higher concentrations of erythromycin increased phagocytic killing. Although the degree of sensitization varied in different strains of streptococci, all strains tested were significantly more susceptible to phagocytic killing after erythromycin exposure. Killing of erythromycin-treated bacteria that occurred in the absence of antibiotic was dependent upon internalization of the bacteria. Thus, the brief exposure of group A streptococci to inhibitory levels of erythromycin increases their susceptibility to phagocytic killing by peripheral blood neutrophils.

Treatment of pneumococcal postsplenectomy sepsis in the rat with human gamma-globulin

A rat model was used to evaluate the possible effect on experimental postsplenectomy sepsis of a human gamma-globulin preparation for intravenous use (Sandoglobulin). Sixty splenectomized male Sprague-Dawley rats were given 3 X 10(3) Streptococcus pneumoniae type 1 intravenously. Twelve of the animals received no treatment and all died, in contrast to 12 sham-operated controls which all survived the challenge. The remaining splenectomized rats were divided into four groups, each consisting of 12 animals. One group was given 120 mg human gamma-globulin twice intraperitoneally (0.3 g/kg body wt), at 18 and 42 hr, after challenge; 10 of the 12 survived, in contrast to none of the 12 in the second group receiving 120 mg human albumin instead of gamma-globulin (P = 0.00003). When the injections were delayed to 24 and 48 hr, 9/12 gamma-globulin-treated animals still survived, in contrast to 0/12 in the albumin group. These findings point to new possibilities for treatment and perhaps prevention of overwhelming postsplenectomy sepsis by administration of high doses of gamma-globulins.

Intravenous administration of human IgG to newborn infants: changes in serum antibody levels to group B streptococci

A human IgG preparation was given intravenously to 36 newborn infants admitted to the neonatal intensive care unit because of suspected septicaemia. IgG was given as a single dose of 0.4 g/kg body weight. Patient serum was obtained immediately before and 30 min after terminating the infusion. Blood was also withdrawn 2 days after giving the IgG in eight of the infants. The sera were tested by radioimmunoassay for IgG antibody levels to surface antigens of group B streptococci (GBS) types Ia, Ib, II and III and to R-protein. The mean increases in anti-type Ia, Ib, II, III and R-protein antibodies 30 min after the end of infusion were 81%, 73%, 49%, 60% and 69% of the preinfusion levels, respectively. This was followed by a rapid decrease during the following 2 days to 25%-32% of the initial increases. Based on the above findings, a controlled trial of passive immunisation in the management of neonatal GBS septicaemia seems justified. The rapid decline in antibody levels would necessitate a second infusion 24 h after the initial immunoglobulin administration if the suspicion of septicaemia persists.

Association between serum inhibitory and bactericidal concentrations and therapeutic outcome in bacterial endocarditis

Several recent reviews on the therapy of bacterial endocarditis have recommended that a serum inhibitory and/or bactericidal concentration (SIC/SBC) of 1:8 or more be achieved to ensure successful therapeutic outcome. We conducted a methodologic and statistical analysis of the available literature on endocarditis to determine the association between SIC/SBC titers of 1:8 or more and therapeutic outcome. We reviewed 17 studies published between 1948 and 1980 in which both SIC/SBC and therapeutic outcome were available. Factors that affect outcome, such as age, duration of symptoms, organism, and valve status, varied widely among the 226 patients. The methods used to measure SIC/SBC differed with respect to the time of obtaining the blood specimen relative to the antibiotic dose, size of the bacterial inoculum, type of broth, and definition of the bactericidal end-point. None of the 17 studies showed a significant association between SIC/SBC titers of 1:8 or more and survival or bacteriologic cure. Fifteen of the 17 also failed to demonstrate a significant association between SIC/SBC titers of 1:8 or more and medical cure. However, each of the studies that failed to demonstrate an association between SIC/SBC titers of 1:8 or more and improved therapeutic outcome had an insufficient sample size to confidently exclude a false-negative result. Analysis of the published data reveals insufficient evidence to demonstrate that SIC and SBC titers are of prognostic value in the therapy of patients with bacterial endocarditis.

The effect of exposure of Staphylococcus aureus to penicillin on susceptibility to the bactericidal activity of human leukocytes

The effect of brief exposure of staphylococci to penicillin upon susceptibility to leukocidal activity was studied. Antibiotic pretreated staphylococci were incubated with normal human leukocytes and viability of the bacteria was determined. Penicillin pretreated staphylococci were more susceptible to killing by leukocytes than untreated control bacteria. The extent of sensitization varied between different strains of staphylococci. When staphylococci were exposed simultaneously to leukocytes and penicillin, the bacteria were protected from the lethal action of penicillin. This study demonstrates that phagocytosed staphylococci are protected from penicillin, but prior exposure to the antibiotic increases their susceptibility to the activity of leukocytes.