She is the foremost authority on Listeria monocytogenes,[2] a deadly and common food-borne pathogen responsible for encephalitis, meningitis, bacteremia, gastroenteritis, and other diseases.

The bacteria is one of the best models of intracellular parasitism because it is particularly hardy, able to survive in a variety of cells, cross multiple host barriers, and spreads through ActA, the protein responsible for actin-based motility.

The discovery by Cossart's lab of the interaction between L. monocytogenes' protein, internalin, and its cell receptor, E-cadherin, was the first such study that successfully demonstrated the molecular mechanism that permits a bacterial agent to cross the placental barrier.

[3] In 2009 Cossart published what she describes as the first "bacterial operon map"—the transcriptional program that regulates Listeria's behavior in different environmental conditions.

[3] As part of her work she has also developed important biological tools, including a transgenic mouse that was the first animal model to overcome bacterial species-specificity.