In vitro effects of cefotaxime and ceftriaxone on Salmonella typhi within human monocyte-derived macrophages

The main objective of this in vitro study was to assess the effects of cefotaxime and ceftriaxone in killing Salmonella typhi in infected human macrophages. Human monocyte-derived macrophages isolated from peripheral blood of human volunteers were cultured in vitro for macrophage differentiation, and subsequently infected with S. typhi strains (a clinical isolate and a standard strain TA-42) at a cell ratio of 10 : 1. MICs of cefotaxime and ceftriaxone were determined by broth microdilution, and the antibiotics were included in the culture medium at one and five times their MIC values. Samples of cell culture medium taken at 0, 3, 6 and 24 h of incubation were cultured for growth of S. typhi on nutrient agar. Gentamicin (10 mg/L) was included in each well except for the control wells, in order to prevent growth of extracellular S. typhi . Both antibiotics showed good in vitro antibacterial effects against S. typhi strains. There were no statistically significant differences between the extracellular and intracellular effects of antibiotics with regard to elimination of the bacteria. Cefotaxime and ceftriaxone are highly effective against extracellular bacterial growth. The results of our in vitro experiments suggest that cefotaxime and ceftriaxone might also be used clinically against susceptible intracellular pathogens such as S. typhi.