Characterization and Quantitation of Experimental Surgical-Wound Infections Used to Evaluate Topical Antibacterial Agents

Abstract

Reproducible experimental surgical-wound infections in mice for use in the evaluation of topical antibacterial agents are described. The experimental wound was created on the backs of mice by means of a midline incision and was infected by means of cotton sutures monocontaminated with Staphylococcus aureus or Pseudomonas aeruginosa. The course of these wound infections was followed by quantitation of surface bacteria through use of a surface rinse technique. Surface wound counts of the infecting organisms thus obtained appeared to reflect the dynamics of the total wound count, as determined by homogenization of biopsied tissue. Treatment of infected wounds with a placebo cream had only a slight effect on surface wound counts and on mortality in the case of the S. aureus infection but enhanced markedly the lethality of the P. aeruginosa infection.The search for new drugs effective as topical antibacterial agents requires the careful and systematic evaluation of drugs not only in vitro but also in vivo, i.e., in the control and management of experimental infections. Several model infections for evaluating topical antibacterial agents have been developed in laboratory animals (5-7, 9, 18, 19, 21) and in humans (3,14,15,20). Although many of the models have proved to be effective in assessing topical agents, most of them do not lend themselves easily to a mass screening of potential agents. This paper describes and characterizes a relatively simple surgical-wound model in mice in which potential topical antibacterial drugs can be evaluated easily and expeditiously against Staphylococcus aureus and Pseudomonas aeruginosa wound infections.MATERIALS AND METHODS Animals. Female CF-1 mice, 18 to 20 g, purchased from a commercial source (Carworth Farms, New City, N.Y.) were used in all experiments. Mice were maintained, 10 to a cage, on wire, in temperatureand humidity-controlled quarters and were allowed food and water ad libitum.Test organisms. S. aureus SC 2406 (phage type 53/77/84/85) and P. aeruginosa SC 8822, clinical isolates from the departmental culture collection, were used as test organisms. The two organisms were maintained on brain heart infusion (BHI) agar (BBL) slants stored at 4 C, with transfers to fresh slants made monthly. For each experiment, a fresh slant was inoculated from the appropriate stock culture and incubated at 37 C for 18 h. After incubation, the growth was removed from the slant by washing with 0.1 M phosphate buffer, pH 7.2, and the resulting suspension was adjusted to a concentration of 2.0 x 109 cells/ml by use of a Klett-Summerson colorimeter with a 540-nm filter and a previously prepared standard curve. The suspension was then appropriately diluted and used for the contamination of sutures or the direct seeding of the experimental wounds. Preparation of contaminated sutures. Commercial cotton thread (Coats and Clark's, mercerized, size 8) monocontaminated with S. aureus or P. aeruginosa was used as the suture material to initiate and potentiate the experimental infection. The thread ...

Efficacy of a New Cream Formulation of Mupirocin: Comparison with Oral and Topical Agents in Experimental Skin Infections

“…The murine model of skin wound infection is established by implanting contaminated sutures (29) and represents the secondary skin infections that may occur following damage by accidental trauma, surgery, and burns or as a result of superinfection of an underlying skin disease. Such foreign-body infections are a stringent test of the efficacy of antibiotics.…”

“…The discriminative models utilized in the studies reported here have been used previously for evaluating systemic and topical antimicrobial agents (5,6,14,29), and the data have been shown to correlate well with efficacy in humans. The results reported here show that the new cream formulation of mupirocin was as effective as the established ointment preparation and, in two of eight trials, it was more efficacious for treating staphylococcal and streptococcal infections.…”

Response of Streptococcus pyogenes to therapy with amoxicillin or amoxicillin-clavulanic acid in a mouse model of mixed infection caused by Staphylococcus aureus and Streptococcus pyogenes

“…The technique used for production of the wound was similar to that described by McRipley and Whitney (15). A length of suture was run under the skin on the back of each mouse under anesthesia, so that a 1-cm length of suture lay between the skin layers and the panniculus carnosus.…”

Responsiveness of Experimental Surgical-Wound Infections to Topical Chemotherapy

“…In the preceding paper experimental surgical-wound infections in mice were described and characterized (6). In addition, a rapid method for quantitating bacteria in the wound infections was presented.…”

“…Details of the experimental Staphylococcus aureus and Pseudomonas aeruginosa surgical-wound infections were presented in the preceding paper (6). aliquots of wound washings and bacterial inocula containing 50 to 300 viable organisms were placed on brain heart infusion agar plates and spread over the surface of the agar.…”