Clinical Isolates of Streptococcus pneumoniae with Different Susceptibilities to Ceftriaxone and Cefotaxime

DOI: 10.1128/AAC.47.10.3155-3160.2003

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FIG. 1.
Scatterplot of ceftriaxone and cefotaxime MICs determined by broth microdilution for 1,000 isolates of S. pneumoniae. The solid line is the linear regression line (r = 0.977). The area between the two diagonal dotted lines contains isolates with ceftriaxone MICs and cefotaxime MICs within ±1 doubling dilution of each other. The vertical dotted lines divide the isolates into ceftriaxone susceptible (MIC, ≤1 μg/ml), intermediate (MIC, 2 μg/ml), and resistant (MIC, ≥4 μg/ml) groups; the horizontal dotted lines divide the isolates into cefotaxime susceptible (MIC, ≤1 μg/ml), intermediate (MIC, 2 μg/ml), and resistant (MIC, ≥4 μg/ml) groups.

TABLE 1.

In vitro susceptibilities of 1,000 isolates of S. pneumoniae to ceftriaxone and cefotaxime tested by broth microdilution and E-test

Test method and antimicrobial agent	Isolate phenotype^a	MIC (μg/ml)				MIC interpretation^b
Test method and antimicrobial agent	Isolate phenotype^a	Range	Mode	MIC₅₀	MIC₉₀	% Susceptible	% Intermediate	% Resistant
Broth microdilution
Ceftriaxone	All isolates	≤0.004-8	0.015	0.015	1	98.6	0.6	0.8
	Penicillin susceptible	≤0.004-0.5	0.015	0.015	0.03	100	0	0
	Penicillin intermediate	0.015-1	0.5	0.25	0.5	100	0	0
	Penicillin resistant	0.12-8	1	1	1	92.6	3.2	4.3
Cefotaxime	All isolates	≤0.004-8	0.015	0.015	1	96.6	1.8	1.6
	Penicillin susceptible	≤0.004-1	0.015	0.015	0.03	100	0	0
	Penicillin intermediate	0.015-1	0.12, 0.5	0.25	1	100	0	0
	Penicillin resistant	0.25-8	1	1	2	81.9	9.6	8.5
E-test
Ceftriaxone	All isolates	0.004-4	0.012	0.016	0.5	98.3	0.8	0.9
	Penicillin susceptible	0.004-0.75	0.012	0.012	0.032	100	0	0
	Penicillin intermediate	0.012-1	0.38	0.19	0.5	100	0	0
	Penicillin resistant	0.25-4	0.75	0.5	1	91.0	4.3	4.8
Cefotaxime	All isolates	0.004-8	0.016	0.023	1	95.8	2.6	1.6
	Penicillin susceptible	0.004-1	0.016	0.016	0.047	100	0	0
	Penicillin intermediate	0.016-1	0.38	0.19	0.75	100	0	0
	Penicillin resistant	0.38-8	1	1	1.5	77.7	13.8	8.5

↵ a Of the 1,000 isolates tested, 666 were penicillin susceptible, 146 were penicillin intermediate, and 188 were penicillin resistant.
↵ b MICs interpreted using NCCLS 2003 guidelines (19).