A novel chromogenic ester agar medium for detection of salmonellae

A novel agar medium, chromogenic Salmonella esterase (CSE) agar, for the di fferentiation of salmonellae is described. The agar contains peptones and n utrient extracts together with the following (grams per liter unless otherw ise specified): 4-[2-(4-octanoyloxy-3,5-dimethoxyphenyl)-vinyl]-quinolinium -1-(propan-3-yl carboxylic acid) bromide (SLPA-octanoate; bromide form), 0. 3223; lactose, 14.65; trisodium citrate dihydrate, 0.5; Tween 20, 3.0; ethy l 4-dimethylaminobenzoate, 0.035% (wt/vol), novobiocin, 70 mg liter(.)(-l) The key component of the medium is SLPA-octanoate, a newly synthesized este r formed from a C-8 fatty acid and a phenolic chromophore. In CSE agar, the ester is hydrolyzed by Salmonella spp. to yield a brightly colored phenol which remains tightly bound within colonies. After 24 h of incubation at 37 or 42 degrees C, colonies of typical Salmonella spp. were burgundy colored on a transparent yellow background, whereas non-Salmonella spp. were white , cream, yellow or transparent. CSE agar was evaluated by using a panel of strains including a high proportion of Salmonella and non-Salmonella strain s giving atypical reactions on other differential agars. The sensitivity (9 3.1%) of CSE agar for non-typhi salmonellae compared favorably with those o f Rambach (82.8%), xylose-lysine-deoxycholate (XLD; 91.4%), Hektoen-enteric (89.7%), and SM ID (91.4%) agars. The specificity (93.9%) was also compara ble to those of other Salmonella media (SM ID agar, 95.9%; Rambach agar, 91 .8%; XLD agar, 91.8%; Hektoenenteric agar, 87.8%). Strains of Citrobacter f reundii and Proteus spp. giving false-positive reactions with other media g ave a negative color reaction on CSE agar. CSE agar enabled the detection o f >30 Salmonella serotypes, including agona, anatum, enteritidis, hadar, he idelberg, infantis, montevideo, thompson, typhimurium, and virchow, which a ccounted for 91.8% of the salmonella isolates recorded by the Public Health Laboratory Service (Colindale, London, England) for 1997.