Factors affecting haemolysin production and Congo red binding in Salmonella enterica serovar Typhimurium DT 98

Differences in haemolysin expression were observed in a strain of Salmonella enterica serovar Typhimurium definitive phage type (DT) 98 cultured under various conditions. Haemolysin expression was optimal in cultures grown micro-aerobically. The zones of haemolysis were wider after longer periods of incubation. Haemolysin production varied after growth in the following media (greatest to least): brain heart infusion (BHI) broth > nutrient broth (NB) > trypticase soy broth (TSB) > M-9 glucose medium. Haemolysin production correlated directly with Congo red binding in nutrient broth. On Congo red blood agar, colonies were smaller, with dark centres and wider zones of haemolysis. Culture-cell-free haemolysin activity was higher, but cell-bound haemolysin activity was very low in growth medium supplemented with Congo red. Boiled tea extract at 25% v/v (of 25% w/v tea infusion) in PBS and nutrient broth was bactericidal to S. Typhimurium DT 98. The addition of boiled tea extract to growth medium inhibited haemolysin production by S. Typhimurium DT 98 at higher concentrations (6–12.5% v/v) but stimulated haemolysin production at lower concentrations (1.5–3% v/v). The pre-treatment of bacterial cell suspensions with lower concentrations of tea extract (1.5–3% v/v) also altered the Congo red binding, which showed an inverse correlation in nutrient broth.