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Correspondence Activity of β-lactams (ampicillin, meropenem), gentamicin, azithromycin and moxifloxacin against intracellular Listeria monocytogenes in a 24 h THP-1 human macrophage model Sir, Listeria monocytogenes is a facultative intracellular organism causing persistent and life-threatening infections. Recurrence and dissemination of the infection is due to the lack of eradica-tion of intracellular forms, particularly from phagocytes. 1 Currently recommended antibacterial therapy of listeriosis combines gentamicin and ampicillin (or meropenem). 2 Yet, gentamicin is inactive against the intracellular forms of Listeria , and ampicillin and meropenem are only bacteriostatic in a short-term (5 h) cell culture model. 3 This suggested to us that mechanisms other than intrinsic antibacterial properties could play a critical role in vivo, or that the model used did not sufficiently take into account the influence of time (a point of particular importance for β-lactams that are time-dependent antibiotics 4). We have therefore examined the activity of ampicillin, meropenem and gentamicin against extracellular and intracellular L. monocytogenes in a 24 h model. For the sake of comparison, we also examined azithromycin (ineffective against intracellular Listeria in a short-term model in spite of a rapid and very large cellular accumulation) and moxifloxacin (markedly bactericidal at 5 h towards both extracellular and intracellular bacteria). 3 Time–kill studies were carried out as described previously. 3 For determination of activity against extracellular bacteria, cultures were used at a density of 10 6 cfu/mL in tryptic soy broth. Antibiotics were then added at a fixed concentration corresponding to the highest serum concentration (C max) currently observed after conventional administration to patients. 3 Incubation was carried out for 5 and 24 h, and cfu were determined by plate assay. As shown in Figure 1(a), ampicillin and meropenem acted very slowly and were only slightly bactericidal (0.8 and 0.7 log cfu decrease) at 24 h. In contrast, gentamicin and moxi-floxacin were highly bactericidal (gentamicin achieving complete eradication within 5 h). Azithromycin slowed down bacterial growth at 5 h but was without effect at 24 h. For measurement of activity against intracellular L. mono-cytogenes, macrophages (5 × 10 5 /mL) were exposed for 1 h to bacteria (2.5 × 10 6 cfu/mL), washed extensively, and incu-bated for 5 and 24 h with or without antibiotics. Cells were collected, washed, lysed in distilled water and the number of viable bacteria determined by plate assay. Cell protein was determined in parallel. As shown in Figure 1(b), ampicillin and meropenem again …

the journal of antimicrobial chemotherapy/journal of antimicrobial chemotherapy

Activity of beta-lactams (ampicillin, meropenem), gentamicin, azithromycin and moxifloxacin against intracellular Listeria monocytogenes in a 24 h THP-1 human macrophage model