Table 4.

Occurrence of cdt-positive E. coli environmental isolates^a

Sewage sample no.	E. coli CFU/ml	% of Cdt+E. colistrainsb	stx gene	cdt gene	Cdt phage	Inductionc4		Induction (OD600)		S. sonnei 866
						− MitC	+ MitC	− MitC	+ MitC	Plate	Hybridization
1	2.4 × 105	0.05	−	cdt-IV	−	8.5 × 102	3.4 × 102	1.481	0.760	−	−
2	8.5 × 104	0.25	−	cdt-IV	−	9.4 × 102	3.9 × 102	1.548	0.406	+	−
3	1.6 × 104	1.18	−	cdt-IV	−	7.3 × 102	9.4 × 102	1.459	0.893	−	−
4	4.5 × 104	0.89	−	cdt-IV	−	2.2 × 103	1.1 × 102	1.415	0.435	−	−
		0.89	−	cdt-I	−	1.4 × 103	8.7 × 102	1.469	1.059	+	−
5	6.0 × 104	0.33	−	cdt-IV	−	5.1 × 103	8.2 × 102	1.413	0.418	−	−

^aThe table gives the percentage of cdt-positive E. coli strains among the total E. coli counts for each sewage sample. The type of cdt and the presence of Cdt inducible bacteriophages were determined from the ODs (absorbance units) of the cultures. Cdt bacteriophages induced from the strains after spot test on S. sonnei were visualized directly on the plate and, after hybridization with the cdtB probe, scored as follows: +, low lytic spot; ++, strong lytic spot; and −, negative. MitC, mitomycin C.

^bThat is, the percentage of cdt-positive colonies among the total E. coli population (CFU/ml) grown in Chromocult agar (indicated in the previous column).

^cExpressed as cdt copies in phage DNA per ml of induced culture. Values were determined by real-time qPCR in phage DNA extracted from the cultures.