Diarrhoea is a common symptom of bacterial infections of the digestive system. That is why it is very difficult to identify the exact source of an infection, but the causes of infection may be different. In this post, we describe the most common bacteria in the digestive tract.

Salmonella spp.

Salmonella spp. include more than 2,500 serotypes, distinguished based on their antigenic structure. Serotypes S. typhi (the causative agent of typhoid fever) and S. paratyphi A, B and C (paratyphoid fever) cause systemic infections (anthroponoses). Serotypes such as S. enetritidis, S. typhimurium, S. agona, S. hadar, S. virchow and many others, cause salmonellosis, which usually occurs in the form of gastroenteritis. Bacteria are most commonly transmitted on meat, milk and eggs.

Clinical symptoms: fever, abdominal pain, acute diarrhoea, nausea, and sometimes vomiting.

Crossing the intestinal barrier: sepsis, pneumonia, meningitis, osteitis, endocarditis, abscesses.

Escherichia coli bacilli with stx1/2 genes

Shigatoxic Escherichia coli, having the stx1 / 2 (STEC) genes, secrete an enterotoxin that causes gastroenteritis in humans. Two basic groups of immunologically non-cross-reactive Shiga toxin, called Stx1 and Stx2, have been distinguished. The Shiga toxin is also the primary virulent factor in the case of enterotoxic E. coli (EHEC) strains. EHEC infection can occur through the consumption of contaminated food or water and from contact with infected animals. EHEC causes bloody diarrhoea and haemorrhagic colitis, a frequent complication of which is a haemolytic uremic syndrome (HUS).

Escherichia coli bacilli carrying the eaeA genes

Enteropathogenic Escherichia coli, having the eaeA genes (EPEC), is the etiological factor of diarrhoea in young children (under 2 years of age) in developing countries – the so-called “Summer diarrhoea in infants”. EPEC strains induce the formation of characteristic histopathological changes within the intestinal enterocytes called the acronym A/E (attaching and effacing). These changes consist in blurring the structure of the microvilli by reorganizing the cytoskeleton of the host cells. The ability to induce histopathological changes in A/E is possible due to the presence of the locus of enterocyte effacement (LEE) in the genome of bacteria. One of the many genes that make up the LEE island, namely the eae gene, codes for the intimin adhesive protein that mediates EPEC cell adhesion to the enterocyte membrane.

Clinical signs: watery diarrhoea possibly with high temperature and vomiting.

Escherichia coli bacilli with lt/st genes

Enterotoxic Escherichia coli, having the lt/st (ETEC) genes, infections with these strains are secretory diarrhoea, most often resolving without treatment. Induced diarrhoea ETEC strains arise as a result of bacterial secreted enterotoxins: thermolabile LT and/or thermostable ST, for which the coding genes are located in a bacterial plasmid. Most infections caused by ETEC strains appear in travellers, which is why they are called “traveller’s diarrhoea”.

The clinical symptoms are similar to those caused by the bacterium Vibrio chlerae, i.e. watery diarrhoea, nausea, abdominal muscle cramps and a slightly elevated temperature.

Escherichia coli sticks bearing the aggR genes

Enteroaggregative Escherichia coli bacilli carrying the aggR genes (EAEC) are the second most common cause of “traveller’s diarrhoea” in developing and developed countries. Defined as E. coli strains, not secreting LT or ST toxins, and characterized by a specific method of adhesion to HEp-2 cells (intestinal epithelial cells) in the form of aggregates, resembling brick piles. The most important virulence factor is the aggR transcription regulator.

Clinical signs: watery, mucoid diarrhoea associated with slightly elevated body temperature.

Escherichia coli O157

Escherichia coli O157 is one of the most important serotypes of E.coli bacteria, causing acute diarrhoea, haemorrhagic colitis and hemolytic uremic syndrome (HUS). It belongs to the pathogenic group of E.coli bacteria known as EHEC or STEC.

Clostridium difficile

Hypervirulent strain of Clostridium difficile. A new hypervirulent strain of C. difficile called NAP1 (ribotype PCR 027) – is the most frequently isolated ribotype in the European population. So far, the physiological factors responsible for the rapid spread of hypervirulent strains of Clostridium difficile remain largely unknown. They are known to possess a binary toxin (ADP-ribosyl transferase) with properties approximately similar to the toxin produced by Clostridium perfringens. Additionally, hypervirulent strains of C. difficile produce an increased amount of toxins A (16-fold) and B (23-fold) due to the presence of mutations in the regulatory gene (tcdC). In addition, this strain shows high resistance to fluoroquinolones, reduced sensitivity to metronidazole, and an exceptional ability to form spores. Infection with this strain is associated with a more severe clinical course, more frequent relapses, and hence increased mortality. The infection most often affects hospitalized patients, mainly in intensive care units, after long-term antibiotic treatment, with neoplastic diseases, and the elderly.

Clinical symptoms: enteritis, damage to intestinal epithelial cells, post-antibiotic diarrhoea.

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