Stephanie F. Loria

We have been pretty biased towards multicellular organisms in the Taxon of the Month posts. But this month, we are doing justice to our single-celled organism friends giving them the recognition they deserve as they are so crucial to the health of all multicellular life. For September, we focus our attention on the bacteria family, Enterobacteriaceae. Enterobacteriaceae are quite diverse and include more than 200 species in 51 genera (Octavia & Lan 2014; Janda & Abbott 2015). All Enterobacteriaceae are gram-negative, meaning that they possess a thin peptidoglycan layer in their cell walls causing them to appear pink after Gram staining (Beveridge 2001). Some well-known Enterobacteriaceae members include the medically important Escherichia coli, Salmonella and Klebsiella (Janda & Abbott 2015). E. coli is an essential human gut bacterium that can also act as a pathogen under certain conditions (Janda & Abbott 2015). Salmonella is notorious for causing illness of the human digestive system, which is sometimes fatal, and is transmitted through food and water contaminated with feces (Janda & Abbott 2015). Klebsiella species are found free-living in soil or water or in vertebrate digestive systems but are also responsible for a number of human illnesses including urinary tract infections and pneumonia.

Bacteria from the gastrointestinal tract of  Narceus americana.  Photo credit to C. Wright.

Bacteria from the gastrointestinal tract of Narceus americana. Photo credit to C. Wright.

Many organisms rely on gut-inhabiting bacteria to assist with the digestion of various foods. For example, detritivores, organisms that eat decaying organic matter in the soil, rely on bacteria for assistance in breaking down hard-to-digest plant material, such as cellulose (Taylor 1982). Many Enterobacteriaceae inhabit animal digestive systems and are known to assist with digestion (Lauzon et al. 2003). For a class project as an undergraduate, a fellow student (C. Wright) and I agar plated the gut contents of a common detritivore, the large North American millipede, Narceus americanus. After sequencing the 16S rRNA gene of the plated bacterial colonies, we discovered several members of Enterobacteriaceae inhabiting this millipede's gut including Bacillus mycoides, Serratia sp. and Enterobacter cloacae. All three of these bacteria were previously known to inhabit animal digestive tracts. B. mycoides was previously found in both the soil (Lewis 1932) and in earthworm guts (Jensen et al. 2003). Enterobacter cloacae is known from plants and insect digestive systems (Watanabe & Sato, 1998). Serratia has been recorded in the digestive tract of flies in the genus Dacus (Lloyd et al., 1986). It is possible that these bacteria are assisting this millipede species digest its food.

Several studies have examined the diversification of Enterobacteriaceae. Research indicates that the evolution of endosymbiotic forms occurred multiple times in this family (Husník et al. 2011). Additionally, many endosymbiotic Enterobacteriaceae coevolved with their hosts (Duchaud et al. 2003; Moran et al. 2005). Given their species diversity and the wide range of hosts they inhabit, Enterobacteriaceae are great organisms to study for understanding selective pressures on symbiotic relationships.


Beveridge, T.J. 2001. Use of the gram stain in microbiology. Biotechnic & Histochemistry 76: 111–118.

Duchaud, E., C. Rusniok, L. Frangeul, C. Buchrieser, A. Givaudan, S. Taourit, S. Bocs, C. Boursaux-Eude, M. Chandler, C. Jean-Francois and E. Dassa. 2003. The genome sequence of the entomopathogenic bacterium Photorhabdus luminescensNature biotechnology 21: 1307–1313.

Husník, F., T. Chrudimský & Václav Hypša. 2011. Multiple origins of endosymbiosis within the Enterobacteriaceae (γ-Proteobacteria): convergence of complex phylogenetic approaches. BMC biology 9: 87.

Janda, J.M. & S.L. Abbott. 2015. The family Enterobacteriaceae. Practical handbook of microbiology (Goldman, Emanuel and L. H. Greens, eds) 307–319.

Jensen, G.B., B.M. Hansen, J. Eilenberg & J. Mahillon. 2003. The hidden lifestyles of Bacillus cereus and relatives. Environmental microbiology 5: 631–640.

Lauzon, C. R., T. G. Bussert, R. E. Sjogren & R. J. Prokopy. 2003. Serratia marcescens as a bacterial pathogen of Rhagoletis pomonella fies (Diptera: Tephritidae). European Journal of Entomology 100: 87–92.

Lundgren, J. G., R. Michael Lehman & J. Chee-Sanford. 2007. Bacterial communities within digestive tracts of ground beetles (Coleoptera: Carabidae). Annals of the Entomological Society of America 100: 275–282.

Lewis, I.M. 1932. Dissociation and life cycle of Bacillus mycoides. Journal of bacteriology 24: 381–421.

Llyod, A.C., R.A.I. Drew, D.S. Teakle & A.C. Hayward. 1986. Bacteria associated with some Dacus species (Diptera: Tephritidae) and their host fruit in Queensland. Australian Journal of Biological Scienes 39: 361–368.

Moran, N.A., J.A. Russell, R. Koga and T. Fukatsu. 2005. Evolutionary relationships of three new species of Enterobacteriaceae living as symbionts of aphids and other insects. Applied and Environmental Microbiology 6: 3302–3310. 

Octavia, S. & R. Lan. 2014. The family enterobacteriaceae. The Prokaryotes: Gammaproteobacteria 225–286.

Taylor, E.C. 1982. Role of aerobic microbial populations in cellulose digestion by desert millipedes. Applied and environmental microbiology 44: 281–291.

Watanabe, K. & M. Sato. 1998. Plasmid-mediated gene transfer between insect-resident bacteria, Enterobacter cloacae, and plant-epiphytic bacteria, Erwinia herbicola, in guts of silkworm larvae. Current Microbiology 37: 352–355.