High level of persister frequency in clinical staphylococcal isolates

BMC Microbiology

Table 2 Minimum inhibitory concentration (MIC) values of biofilm forming and non-forming isolates as detected by TCP method

Antibiotics	Biofilm			Antibiotics	Biofilm
Antibiotics	Positive (n = 86)	Negative (n = 289)	P value	Antibiotics	Positive (n = 86)	Negative (n = 289)	P value
Oxacillin				Tetracycline
Range	0.125–64	0.125–64		Range	0.125–8	0.125–8
MIC₅₀	1	1		MIC₅₀	0.5	0.5
MIC₉₀	64	64		MIC₉₀	2	8
% resistance	65 (37.4%)	93 (46.3%)	*	% resistance	8 (4.6%)	22 (10.9%)	0.076
Vancomycin				Clindamycin
Range	0.125–8	0.125–8		Range	0.125–1024	0.125–1024
MIC₅₀	2	2		MIC₅₀	0.125	0.125
MIC₉₀	4	4		MIC₉₀	256	128
% resistance	-	-	*	% resistance	32 (18.4%)	36 (17.9%)	0.479
Cotrimoxazole				Penicillin
Range	0.5/9.5–16/304	0.5/9.5–16/304		Range	0.125–8	0.125–8
MIC₅₀	2/38	16		MIC₅₀	0.25	0.5
MIC₉₀	16/304	512		MIC₉₀	8	4
% resistance	78 (44.8%)	76 (37.8%)	0.265	% resistance	132 (75.9%)	163 (81.1%)	0.017
Erythromycin				Ciprofloxacin
Range	0.125–1024	0.125–1024		Range	0.125–256	0.125–256
MIC₅₀	16	64		MIC₅₀	8	8
MIC₉₀	1024	1024		MIC₉₀	128	128
% resistance	102 (58.6%)	139 (69.2%)	0.375	% resistance	104 (59.8%)	115 (57.2%)	0.985
Chloramphenicol				Gentamicin
Range	0.125–32	1–32		Range	0.125–128	0.125–128
MIC₅₀	4	4		MIC₅₀	1	2
MIC₉₀	8	8		MIC₉₀	64	64
% resistance	7 (4.0%)	12 (6.0%)	0.028	% resistance	81 (46.6%)	98 (48.8%)	0.225

The MICs values of the isolates were determined by the agar dilution method. The sensitivity towards each antibiotic was compared between biofilm forming and non-forming isolates. The Chi-square test was used to evaluate the statistical significance of differences in the results. The p value of < 0.05 was considered statistically significant.
*The resistant range of MIC values of oxacillin and vancomycin is different for S. aureus and CNS. For this reason, p value is not mentioned in the table. The sensitivity of oxacillin (n = 45, 58.4%) in biofilm producers (n = 77) vs sensitivity (n = 38, 45.2%) in biofilm non-producers S. aureus was not significantly different (p = 0.542). Whereas the sensitivity of oxacillin (n = 76, 84.4%) in biofilm producers vs. sensitivity (n = 96, 77.4%) in biofilm non-producers CNS were significantly different (p = 0.086). Irrespective of whether biofilm producers or not, all the isolates were vancomycin susceptible (Supplementary Table 1).

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