Tolerance of Plesiomonas shigelloides to pH, Sodium Chloride and Temperature

Abstract

MATERIALS AND METHODS Forty strains of Plesiomonas shigelloides representing environmental and clinical isolates were characterized as to their tolerance to sodium chloride, acid and alkaline conditions and growth temperatures. In Tryptone broth all isolates grew in 3% but not 4% sodium chloride; whereas, in Trypticase Soy broth all isolates grew in 4% and 65% of them grew in 5% sodium chloride. All isolates initiated growth at pH 4.5 and 8.5 and 58% of the isolates grew at pH 4.0. Only 22% of the isolates grew at 8°C and 25% of them grew at 45°C. Heating at 60°C for 30 min destroyed all isolates tested. Plesiomonas shigelloides has been implicated in several outbreaks of gastroenteritis involving water {3,16) and food, such as oysters (5,75) crab (4) and fish {6,17). Although the organism has often been isolated from patients with diarrhea, it can also be isolated from people having no apparent symptoms (10,18). P. shigelloides is considered an opportunistic pathogen, and whether it causes an infection in a person depends on his/her susceptibility or general health at the time of exposure. Groups at highest risk are the elderly, the very young and people who are debilitated (8,9,11). In addition to being isolated from stool samples, P. shigelloides has been isolated from a variety of animals and aquatic environments. These include aquatic animals such as oysters, crabs and fish (1,7,16) as well as cats, dogs, zoo animals and domestic cattle (1,2,14). To better understand the behavior of P. shigelloides in foods, a series of experiments was conducted to determine the affect of pH, sodium chloride concentration and temperature on growth. 'Florida Agricultural Experiment Station Journal Series No. 7178. Forty P. shigelloides isolates obtained from aquatic environments in the Gainesville and Suwannee River, FL areas, humans with diarrhea (Health and Rehabilitative Services, Tal-lahassee, FL) and the American Type Culture Collection #14029, were evaluated for their tolerances to various growth conditions. Table 1 indicates the sources of the organisms. The tolerance tests were conducted at 35°C and were allowed to incubate up to 14 d. If possible, the tolerance range was determined so that the lowest and highest values used gave negative results. Sodium chloride (NaCl) tolerance was tested in two difference broths, Tryptone broth (1% Tryptone in distilled water) and Trypticase Soy broth (TSB). Concentrations of NaCl from 0 to 6% were tested. Tolerance to acid and alkaline conditions was also evaluated using TSB adjusted with 2 N hydrochloric acid or sodium hy-droxide. The pH range tested was 3.5 to 9.5. Temperature (5 to 50°C) of growth was determined in TSB. In addition to the previous growth characteristics, the effect of pasteurization on Plesiomonas was assessed. Six isolates of P. shigelloides (numbers 1,12,21,29,32 and 39; Table 1) were subjected to pasteurization conditions of 60°C for 30 min in TSB. The experiment was conducted in a covered waterbath to ensure constant conditions, and tubes of TSB were pre-warmed to eliminate temperature come-up time. The six cultures were grown in TSB for 24 h at 35°C, then 1.0 ml was transferred to duplicate 9-ml tubes of fresh TSB (prewarmed in the 60°C waterbath). One tube was immediately diluted and plated on Plate Count agar to obtain an initial count. The other tube was heated for 30 min, cooled and then plated on the same medium. Incubation was for 72 h at 35°C (16). RESULTS AND DISCUSSION The tolerance of P. shigelloides to different growth conditions is shown in Table 2. The three parameters tested were concentration of NaCl, pH and growth temperature. TSB was used to grow the isolates because it is a nutritious medium and the results using this medium