Differences in haemolysin expression were observed in a strain of 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 . Typhimurium DT 98. The addition of boiled tea extract to growth medium inhibited haemolysin production by . 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.


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