Dr. Kim Cowles’s paper, Established Pseudomonas syringae pv. tomato infection disrupts immigration of leaf surface bacteria to the apoplast, was published in Frontiers in Microbiology today.  Normally, the interior of a leaf, the apoplast, is usually off-limits to surface inhabiting bacteria. However, we previously demonstrated that leaves infected with Xanthomonas hortorum pv. gardneri (Xhg) induce water soaking of the apoplast which leads to entry, survival, and proliferation of unwanted (from the human perspective) bacteria such as Salmonella enterica in the leaf apoplast. Kim investigated whether this microbe helping microbe was common among phytopathogens that infected leaves. She chose to examine another bacteria, Pseudomonas syringae pv. tomato (Pst) to see if the outcome for S. enterica would be similar. She found that even though this bacteria causes similar symptoms as Xhg, Pst actually prevents S. enterica from colonizing the apoplast. This blocking effect by the resident Pst also applies to any bacteria attempting to immigrate from the leaf surface to the apoplast, including genetically identical Pst.