Estimate of naturally-occurring sand fly gut bacteria. (A) and Serratia marcescens (B) in Leishmania infected flies after oral challenge with Serratia. Estimated as CFUs present in individual Lu. longipalpis midguts either uninfected (Blood + Serratia) or infected with L. mexicana (Leish + Serratia) at 3 days after daily oral challenge with Serratia marcescens via cotton wool (5.7 × 107Serratia CFUmL-1, resuspended with sterile 20% w/v sucrose solution). Asterisk represents statistical difference using Mann-Whitney U test (P ≤ 0.03) of at least two independent experiments.