Bacteria 101: How do pathogens make us sick?

Equipment in Science Research LabHere at Neogen blog, we talk a lot about pathogens.

But what makes them tick? And what makes them pathogens?

A pathogen is an agent such as a bacterium or virus that causes illness. When it comes to foodborne illness, this includes Salmonella, E. coli, Listeria, and norovirus, for example. For now, we’re going to stick to bacterial pathogens as these include some of the most well-known culprits for foodborne illness outbreak (including the first three listed above).

Each type of bacteria differs in the type of illness it causes, how it does so and the severity of the illness. Characteristics that allow bacteria to cause illness are called virulence factors, which allow the bacteria to infect and live within its host by avoiding the defense mechanisms sent to destroy it, such as the body’s immune system1.

These factors can include physical structures on the bacterial cell itself, such as flagella (a thread-like tail that helps the bacteria move), and  fimbriae (pili), which are hair-like structures on bacterial cells that help the cell to stick to the host’s body. Other bacteria produce toxins or actually invade the body’s cells2, according to The Virtual Museum of Bacteria.

So, which virulence factors do some of the most common foodborne illness causing bacteria use to make people sick? Let’s break it down by pathogen:




While there are many different types of Salmonella, two common types implicated in food poisoning are Salmonella Typhimurium and Salmonella Enteritidis. Once consumed, the bacteria make their way to the gut, where it moves from the lumen to the small intestine3. Once in the small intestine, Salmonella enter enterocytes, which absorb nutrients and water, and other cells, using a variety of virulence factors many involving the proteins T3SS1 and T3SS2, which help Salmonella invade the cell3. This leads to inflammation of the gut and symptoms typical of a foodborne Salmonella illness, including diarrhea, fever, headache and lethargy, according to the U.S. Food and Drug Administration’s  (FDA) The Bad Bug Book.

E. coli (STEC)

E. coli

E. coli

Many types of E. coli produce toxins that can cause severe illness. In the U.S., Shiga toxin-producing E. coli, or STECs, have been in the cross-hairs as of late (last year, six STECs joined the most well-known E. coli strain, O157:H7, as being listed as adulterants). In fact, E. coli O157:H7 causes roughly 75 percent of global enterohemorrhagic E. coli (EHEC) cases, the Bad Bug Book notes.

Shiga toxin produced by certain strains of E. coli can cause bloody diarrhea, cramping, nausea or vomiting, clotting issues and, in the most severe cases a kidney disease called hemolytic uremic syndrome (HUS) can occur or death. (For a summary of E. coli’s virulence factors, click here).




Although Listeria isn’t one of the most common foodborne pathogens, it is one of the most dangerous. Listeria monocytogenes has a case fatality rate of about 15 to 30 percent and causes about 250 illnesses on average in the U.S., according to the Bad Bug Book.

Listeria causes nausea, vomiting, fever and sometimes diarrhea. However, pregnant women and their babies are especially susceptible to Listeria; sadly, Listeria can be fatal to babies in the womb (about one-third of cases end in failed pregnancies). Additionally, if Listeria infection spreads to the bloodstream the fatality rate spikes to up to 70 percent for listerial meningitis and 50 percent from septicemia.

Listeria causes illness by entering the host’s monocytes, macrophages or polymorphonuclear leukocytes (these are different types of cells) using an invasin, which is a protein that helps Listeria enter the cell4. From there it reproduces and spreads. This process is aided by proteins on the bacteria’s surface that make it easier for it to spread cell-to-cell and help it live within other cells, according to the Bad Bug Book. Listeria also is aided by several virulence factors, such as its ability to grow at lower temperature, its ability to grow “actin tail” used to help move around and into the cell, and its ability to stick to and enter cells4.



1 Cross, A. (2008). What is a virulence factor?Crit Care,12(6), 196. doi: 10.1186/cc7127

2 Wassenaar, T. (2009, January 06). Bacterial pathogenicity. Retrieved from

3 Cardenal-Muñoz E, Ramos-Morales F (2011) Analysis of the Expression, Secretion and Translocation of the Salmonella enterica Type III Secretion System Effector SteA. PLoS ONE 6(10): e26930. doi:10.1371/journal.pone.0026930

4 Todar, K. (2012). Listeria monocytogenes.Todar’s online textbook of bacteriology. Madison: Retrieved from

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