Hello and welcome to a new episode of the medical microbiology series. This video is about the Enterobacteriaceae family – the largest group of gram-negative bacilli and how to accurately differentiate them from other gram-negative bacteria.
Enterobacteriaceae is an important family because it includes more than 30 genera and more than 110 species, it’s also medically important containing well known bacteria such as E. coli, Klebsiella, Enterobacter, Salmonella, Shigella and Proteus. Other species were previously classified under this family, like Yersinia which causes bubonic plague. This organism killed 1/3 of the population during the 14th century, but since 2016 Yersiniaceae have been classified as a separate family but are still included in the Order Enterobacterales which also include Enterobacteriaceae.
Enterobacteriaceae species all share some genera characteristics – they inhabit the intestinal tract as normal flora but may also be pathogenic in both humans and animals. This preference for the intestinal tract is what gave the family its name, which is derived from words meaning enteric bacteria.
Another characteristic of this family is that they are all gram-negative bacilli which appear as pale red rods after gram staining. They are also facultative anaerobes which means that they can survive in the presence or absence of oxygen. They are also oxidase negative and are generally able to ferment sugars, specifically glucose, but not all species can ferment lactose which is considered a critical differential factor in Enterobacteriaceae identification which effectively divides the family into two large groups. These are the coliforms that can ferment lactose such as E. coli, Klebsiella and Enterobacter, and the non-lactose fermenters like Salmonella, Shigella and Proteus.
This is the explanation for the two different culture media types that are identical in everything except their sugar content – this media is Violet Red Bile Glucose agar (VRBG), or VRBL for Lactose. VRBG is selective for all members of Enterobacteriaceae, while VRBL is differential for the lactose fermenting coliforms. Lactose can also be added to other gram-negative selective culture medias such as MacConkey, CLED, Eosin Methylene Blue and Triple Sugar Iron Agar.
Pseudomonas aeruginosa is a bacterium that shares many similarities with the Enterobacteriaceae but does have some key differences which include being an obligate aerobe, oxidase positive and non-fermentative of carbohydrates except glucose which it can utilise via an oxidative pathway. The similarities include being positive for nitrate reduction and motility (with the exception of Klebsiella and Shigella), they are also tolerant to bile salts and crystal violet. Media that contains bile salts and crystal violet like MacConkey agar and VRBG are selective for this and other gram-negative bacteria by inhibiting the growth of gram-positive organisms.
Infections caused by Enterobacteriaceae mostly occur in the gastrointestinal and urinary tracts and can lead to struvite stones in the kidneys or bladder as the enteric bacteria such as Proteus or Klebsiella invade the urinary tract and start to produce urease enzymes. Other diseases can be caused by this family when the organisms invade the blood and travel around the body, including Septicaemia, Meningitis, Endocarditis, Pneumonia or Liver abscesses.
Enterobacteriaceae are classified according to their pathogenicity as being either obligate pathogens such as Salmonella, Shigella, some E. coli and Klebsiella, opportunistic pathogens like E. coli, Klebsiella, Enterobacter, Proteus, Serratia and Morganella, or non-pathogenic like Citrobacter or Hafnia.
They may also be classified according to their antigenic structure. There are three main antigenic structures in this family O, H and K. Where the O antigen is a heat stable lipopolysaccharide in the cell wall, the H antigen is a heat labile protein present in the flagellated or fimbriated bacteria (often referred to as the flagellar antigen) and the K antigen is found in encapsulated bacteria. These antigens differ from species to species or even subspecies to subspecies. The antigenic structure can be identified using latex identification or serological tests that use antibodies with opposing structure such as the Widal test which detects Typhoid caused by Salmonella through the O and H antigens.
Other forms of classification utilise the previously mentioned lactose fermentation and can be further distinguished into groups based on the speed of this reaction. Some bacteria are referred to as rapid lactose fermenters like E. coli, Klebsiella and Enterobacter while others may be late or slow fermenters like Citrobacter and Serratia. Enterobacteriaceae that cannot ferment lactose such as Salmonella, Shigella and Proteus are simply referred to as non-lactose fermenters.
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