Adjunctive Clavulanic Acid Abolishes the Cefazolin Inoculum Effect in an Experimental Rat Model of Methicillin-Sensitive Staphylococcus aureus Endocarditis

William R. Miller; Kavindra V. Singh; Cesar A. Arias; Barbara E. Murray

doi:10.1128/AAC.01158-18

DOI: 10.1128/AAC.01158-18

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FIG 1
Time-kill curve of S. aureus strain TX0117 at standard and high inocula. At a standard inoculum, growth in the presence of CFZ 64 μg/ml resulted in an approximately (3.5 ± 0.8)-log₁₀ reduction in CFU/ml at 24 h. At a high inoculum, growth in the presence of CFZ 64 μg/ml or AMC 1/0.25 μg/ml showed similar colony counts to the antibiotic-free control. The combination of CFZ 64 μg/ml plus AMC 1/0.25 μg/ml resulted in a reduction of 3 ± 0.4 log₁₀ CFU/ml at 24 h. These results are geometric means from three independent experiments; error bars represent the standard deviations. AMC, amoxicillin-clavulanate; CFZ, cefazolin.
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FIG 2
Clavulanic acid abolishes the cefazolin inoculum effect in a rat endocarditis model. (A) S. aureus TX0117 treated with CFZ plus CLA, CFZ alone, or untreated control animals (T = 0). Animals treated with CFZ plus CLA showed an additional mean reduction of 5 ± 0.7 log₁₀ in CFU/g of vegetation compared to CFZ alone after 72 h of therapy. (B) S. aureus TX0117c without BlaZ activity, treated with cefazolin alone showed a mean reduction of 6.5 ± 0.6 log₁₀ CFU/g compared to that of the untreated Bla-cured infected controls (T = 0), a decrease comparable to the parental strain treated with the cefazolin plus clavulanic acid combination (decrease of 7.1 ± 0.5 CFU/g). CLA, clavulanic acid; CFZ, cefazolin; βla+, β-lactamase positive; βla-cured, no β-lactamase activity; SD, standard deviation.

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TABLE 1

S. aureus MICs at standard and high inocula

Treatment^a	MIC (μg/ml)
Treatment^a	ATCC 25923 (βla⁻, CIE⁻)	ATCC 29213 (βla⁺, CIE⁻)	TX0117 (βla⁺, CIE⁺)
Standard inoculum (10⁵ CFU/ml)
AMC	0.125/0.031	0.25/0.063	0.5/0.125
SAM	ND^b	0.125/0.063	0.5/0.25
TZP	ND	1/0.125	1/0.125
Cefazolin	0.5	0.5	1
High inoculum (10⁷ CFU/ml)
AMC	0.125/0.031	1/0.25	8/2
SAM	ND	2/1	4/2
TZP	ND	1/0.125	4/0.5
Cefazolin	0.5	4	128
Cefazolin combinations (μg/ml)^c
Cefazolin plus AMP (10)	ND	4	64
Cefazolin plus AMC (10/2.5)	ND	<0.125	<0.125
Cefazolin plus AMC (1/0.25)	ND	<0.125	0.5
Cefazolin plus AMC (0.5/0.125)	ND	0.5	1
Cefazolin plus AMC (0.25/0.063)	ND	0.5	1
Cefazolin plus SAM (0.5/0.25)	ND	ND	2
Cefazolin plus TZP (1/0.125)	ND	ND	1

↵a AMC, amoxicillin-clavulanic acid; SAM, ampicillin-sulbactam; TZP, piperacillin-tazobactam; AMP, ampicillin.
↵b ND, not done.
↵c MICs for cefazolin combinations reported as MIC of cefazolin in the presence of specified concentrations of AMP, SAM, TZP, or AMC.