Intra- and extracellular activity of dicloxacillin and linezolid against small colony variants of Staphylococcus aureus in vivo

Abstract number: P1907

Sandberg A., Hughes D., von Eiff C., Skov R.L., Frimodt-Møller N.

Objectives: The Small Colony Variants (SCV) phenotype of S. aureus has been observed in connection with chronic and recurrent infections, and intracellular persistence of SCVs is suspected. The aim was to compare the virulence of the wild type and the SCV phenotypes and their response to Dicloxacillin (DCX) and Linezolid (LNZ) extra- and intracellularly in vivo

Methods: A clinical menadione auxothrophic SCV isolate (OM 1b) and the corresponding wild type (WT) isolate (OM 1a) from a patient with chronic infection were tested in the mouse peritonitis model. The two phenotypes were clonally identical by PFGE. Both phenotypes were DXC and LNZ susceptible.

In vivo studies: After i.p. inoculation in mice of 2.5×107 CFU S. aureus (OM 1a or OM1b) in 5% mucin either one dose of DXC (1.5 mg/mouse), LNZ (0.5 mg/mouse) or vehicle was given sc 2 h later.. Four and 24 hr after treatment, mice were euthanised; a peritoneal wash in saline was performed and sampled.

Ex vivo separation assay: The total bacterial count (Total) was quantified before divid-ing the sample into two identical fractions. The extracellular bacterial count (Extra) was determined in the supernatant after centrifugation of one of the sample fractions. In the other sample fraction, extracellular bacteria were killed by adding lysostaphin to a final concentration of 15 mg/ml. The cells were lysed in water after lysostaphin wash out and the intracellular count (Intra) quantified. Groups were compared using the Mann-Whitney test.

Results: As shown in the table, CFUs for untreated Total WT vs SCV were not different after 2 h but significantly lower for SCV after 6 h (P = 0.0023) indicating slower growth in vivo. Similar lower CFUs were seen for SCV extra- (P = 0.03) and intracellularly (P < 0.0001). Both antibiotics displayed activity both extra- and intracellularly, but the reduction in CFUs were lower for the SCV than for the WT in all compartments. DXC showed 0.5–1 logCFU better killing activity than LNZ overall.

Conclusion: Both the WT and the SCV variant of S. aureus resulted in intracellular in-fection, however, already after 6 h the SCV showed significantly slower growth than the WT in vivo. The lower activity in vivo of the two antibiotics against the SCV was probably a result of the slower growth. Both antibiotics were active extra- as well as intracellularly with DXC displaying somewhat better killing activity in both compart-ments.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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