A Student’s t-test was used to assess if the colocalization level was significantly different from that of LVS. Transmission electron microscopy Protocol for infection and sample preparation for TEM has been described elsewhere [17]. Sections were viewed with a JEOL JEM 1230 Transmission Electron Microscope (JEOL Ltd., Tokyo, Japan). The membrane integrity was scored by counting at least 100 bacteria from each sample and categorizing Danusertib price each as having: (i) an intact phagosomal membrane,

(ii) a slightly damaged phagosomal membrane (< 50% of membrane integrity affected), (iii) a highly damaged phagosomal membrane (> 50% of membrane integrity affected), or (iv) S63845 mouse little or no residual membrane (cytoplasmic). Intracellular replication in macrophages Cells were infected with indicated MOI and the infection was

allowed to proceed for 2 h followed by washing and addition of fresh cell medium containing 5 μg/ml gentamicin. The number of viable intracellular bacteria at different time points was determined by lysing the monolayers in PBS supplemented with 0.1% deoxycholate and plating serial dilutions on modified GC-agar base plates. A two-sided Student’s t-test was used to determine whether the growth of a strain differed significantly from AMN-107 order that of LVS. RT-qPCR on intracellular bacteria After infection, J774 murine macrophages were lysed at various time points, by adding one ml Trizol reagent (Ambion, Austin, TX, USA) to each well and scraping with a pipette tip. The suspension was transferred to a 2.0 ml tube and further sample preparation was performed as described earlier in the section “Reverse transcriptase quantitative PCR”. PCR amplification of the 16S

gene of F. tularensis was used as a measure of the number of bacteria, primer sequences have been published elsewhere [31]. Mouse infections In order to determine the virulence of F. tularensis strains, groups of C57BL/6 J female mice (n = 5) were infected intradermally with indicated bacterial doses and mice were examined also twice daily for signs of illness, and euthanized by CO2 asphyxiation when they showed signs of severe illness, indicating that they were less than 24 h from death. The number of viable bacteria was determined by homogenizing spleens in PBS and plating on GC-agar. All animal experiments were approved by the Local Ethical Committee on Laboratory Animals, Umeå, Sweden (no. A113-08). LDH release assay The LDH release assay has been described in detail elsewhere [17]. In short, cells were infected as described in “Cultivation and infection of macrophages”, at an indicated MOI, washed and new medium added 30 min prior to sampling. Supernatants were collected at indicated time points, and the relative amount of released lactate dehydrogenase was determined using a Cytotox 96 kit (Promega, Madison, WI) according to the manufacturer’s instructions.