COMPARAÇÃO ENTRE O CALDO TRIPTOSE-LAURIL E O CALDO LACTOSADO, NA DETERMINAÇÃO DO NÚMERO DE BACTÉRIAS COLIFORMES NAS ÁGUAS DAS PRAIAS DOS MUNICIPIOS DE SANTOS E SÃO VICENTE

Abstract

The use of lauryl-tryptose broth in the presumptive test of the bacteriological examination of water has been found to increase the number of coliform isolations. By reducing the number of false positives it affords also appreciable savings in time and matérial to the laboratories concerned with the control of the sanitary quality of water for drinking purposes. Since that culture medium inhibits the growth of spore·bearing bacteria, which in sea water are likely to survive to a greater extent than colifirmes, one wonld expect that in the examination of sea water the· medium should preserrt a more pronounced advantage than for fresh water. In the present work, the efficiency of lauryl-tryptose broth was compared with that of lactose broth in the examination of water from Santos, São Vicente and Praia Grande beaches. Duplicate tests were made with 104 samples (36 from Santos, 28 from São Vicente and 40 from Praia Grande) in both media. In each test 3 portions were inoculated for each of 4 dilutions, making up a total of 2,496 plantings in each medium. 'Vhenever the presumptive tests were positive, their confirmation was sought in new tests perform~d in: brilliant-green-lactose-bile broth. All techniques were followed according to the standard methods of the A.P.H.A. In the presumptive tests, laury-tryptose broth gave 7.6 and 4.9 per centless positive portions than lactose broth, after 24 and 48 h, respectively. In the confirmed tests, however, the number of positive portions was also smaller fDr lauryl-tryptose broth than fDr lactose broth, 3.6 and 3.8 per cent less portions being confirmed for 24 and 48 h positive presumptive tests, respectively. Analogous results are observed when the data corresponding to Santos, São Vicente and Praia Grande are considered separately. The results obtained with the two culture media were also compared in terms of the "most probable numbers", as summarized in table VII, in which the geometric means of the MPN values are presented separately for the presumptive and confirmed tests of the Santos, São Vicente and Praia Grand beaches. In that ta ble, data for Praia Grande are also presented in two groups of sampling points, G 1 to G5 and G6 to G 10. It can be concluded that: (a) in the water of the beaches studied, the use of lauryl-tryptose broth did not show any advantage over that of lactose broth; (b) the number o f confirmed portions was smaller for lauryl tryptose broth, a disadvantage in relation to lactose broth that appeared to be accentuated when the results are analysed by the "most probable numbers"; (c) the efficiency of lauryl-tryptose broth as a presumptive medium in the examination of these waters might be influenced by the degree or possibly the age of contamination, since the inferiority of this medium seemed intensified for the sections (G1 to G5) of Praia Grand nearest to the sewage outfall from the cities of Santos and São Vicente.