Lecture 3 Flashcards
Order: Enterobacterales
General morphological and biochemical characteristics
- 29 genera including type genus Escherichia, as
well as other genera e.g. Salmonella, Klebsiella,
Enterobacter - Optimum temperature 37 °C- suggests they reside in mamalian hosts
- Gram-negative, non-spore forming rods
- Facultative anaerobes- can grow in both oxygen and the absence of oxygen but get more energy in aerobic conditions so prefer it
- Catalase positive
- Oxidase negative- lack cytochrome c
- Nitrate reductase positive
- Motile, via peritrichous flagella (a few exceptions)
- DNA genome characteristics: G+C content 38-60%; genome size ~5 M bases- large genome for bacteria
Enterobactericeae: common aspects
- Habitats: gastrointestinal tract of hosts including humans, animals and insects
- Widespread contamination of environment: sewage, soil, water, plants, food- found in plants as we use animal faeces for fertalising
- Routes of infection: oral, via wounds, urinary tract, respiratory tract
- Disease: diarrhoea, sepsis, urinary tract, CNS and brain
- Among the most pathogenic and most often
encountered organisms…….
Food-borne illness
alone affects 1
million people
each year in the
UK
Half of all women experience
a UTI in their lifetime; ~70% of
UTIs are caused by E. coli
- E.coli is a dominant aerobic organism in the gut but represents a small proportion of the microbiome in the gut
Classification of Salmonella species is complex
why?
- 2 species
7 subspecies
>2,600 serovars - Some serovars are
host-restricted e.g.
Typhi (human);.
Abortusovis
(sheep) - Most serovars
infect a wide range
of hosts e.g.
Typhimurium - serovars are like a group; they are being classified on their surface antigens
- flagella is an antigen, LPS is another antigen and capsule is another antigen, these antigens are how we differentiate the salmonella serovars
how were species originally defined?
Species and subspecies were originally defined by DNA-DNA hybridisation, confirmed by MLEE and MLST and are currently differentiated by biochemistry and serology
what is the difference between typhoidal and non-typhoidal salmonella?
The split in typhoidal and non-typhoidal is based on the disease syndrome. (typhoidal) Typhoid fever and paratyphoid fever is prolonged,
(non-typhoidal) whilst extra-intestinal infection is usually acute and metastatic. Gastroenteritis is characterised by diarrhoea.
Typhoidal Salmonella: Causes systemic illness (typhoid fever), primarily transmitted via the fecal-oral route, with humans as the main reservoir.
Non-Typhoidal Salmonella: Causes gastrointestinal illness (salmonellosis), primarily transmitted through contaminated food, with various animals as reservoirs.
how are serovars differentiated?
Differentiation of serovars is by agglutination with specific antisera against LPS (O), two phases of flagella (H1 and H2). There are 46 0 antigen, 85 H antigen and 1 capsule antigen (Vi/k) antigen which have been described in about 1. 500 combinations within subspecies 1.
Genus Salmonella:
differentiation tests
- Non-lactose fermenter (E. coli ferments lactose but Shigella does not)
- Indole test negative (E. coli positive; Shigella variable)
- Various selective medias can distinguish
Salmonella from E. coli or Shigella by: - H2S production
- Acid production during
carbohydrate fermentation
e.g. XLD media, SS media - different biochemical properties as a result can help us differentiate between different bacteria
on XLD media
E. coli – yellow
Salmonella – black
Shigella - red
Typhoidal Salmonella : impact & disease
Typhoid fever (S. Typhi)
~ 15 million new cases each year, with
about 1% deaths
1st phase:
slow fever, rose spots, mild, bacteremia
2nd phase:
organism reaches gallbladder, formation of
ulcers, haemorrhage, death (20%)
Typhoid state “muttering delirium” or “coma
vigil” (picking at bedclothes and imaginary
objects)- has neurological impact
Enteric fever (S. Paratyphi)
→ similar to typhoid fever but less severe; rare
Non-typhoid Salmonella (NTS): impact
-often a foodborn illness
- Global burden ~94 million cases (155,000 deaths) each year, of which
about 80 million were estimated as foodborne origin - UK data: 8-9,000 confirmed cases per year- not likely to cause death
- Predominant serovars:
Salmonella Enteritidis
Salmonella Typhimurium
Salmonella Heidelberg
Salmonella Newport - NTS causes self-limiting enteritis (inflammation of the small intestine)
in healthy individuals- as most individuals’ immune systems can contain the bacteria - Can be invasive (iNTS) in regions
with immunocompromised /
malnourished individuals e.g. in
sub-Saharan Africa
NT Salmonella: infection and
pathogenesis
Despite differences in disease outcome, all Salmonella must cross epithelial barrier to colonise the host
* cross epithelium via phagocytic cells (M cells, DCs) or direct uptake; can g through a vacuole
* targets macrophages, or re-invades epithelial cells from basolateral side
* may evade killing by inducing macrophage apoptosis / manipulate for bacterial replication
* severe disease results from systemic spread and bacteraemia
However a healthy individuals immune system can clear this disease
Main Salmonella virulence factors:
Type 3 secretion systems (T3SSs) – encoded on Salmonella
pathogenicity island (SPI)
T3SS attaches to a host cell and injects effectors into the cell- salmonella has 2 used at different points in the salmonella virulance cascade
first T3SS = extracellular ‘molecular syringe’ that is positioned across the inner and out membrane of salmonella and that transfers
proteins (effectors)
from bacterial
cytoplasm to host cell these then manipulate the host cell causing an axon rearrangment that results in the bacterium being taken up by host
once inside the host cell the salmonella remians in a vacuole that would usually cause death however the salmonella uses a second type 3 secretion system and a different set of effectors/proteins to control the environment of the vacuole
the T3SSs are located at different points of the genome at different SPIs
SPI1 – encodes genes
necessary for invasion
of intestinal epithelial
cells and induction of
intestinal secretory and
inflammatory responses
- triggers- cell entry, apoptosis, loss of electrolytes and inflammation
SPI2 – encodes genes
essential for intracellular replication and
necessary for
establishment of
systemic infection
beyond the intestinal
epithelium
- triggers- systemic spread, proliferation in host organs and intracellular proliferation
Vehicles of NTS Salmonella infection (UK)
- Route: faecal-oral transmission
- Predominantly poultry and poultry products but many foods have been
associated with infection - UK: pre-1980s Eggs! after 1980s a vaccine was created to prevent the bacteria
- UK: meat, milk,
sausages and even
chocolate!
-5/5/22 – 101 cases
linked to ‘Kinder’
outbreak, most in
children < 5 years old
Genus: Escherichia
E coli can be typed based on its surface antigens same surface antigens as salmonella such as the O,H and K antigen
- Escherichia genus – E. coli first isolated 1919, Theodor Escherich
- Five species: E. albertii, E. coli, E. fergusonii, E. hermannii, E. vulneris
- E. coli colonises mammalian GI tract a few hours after birth and maintains
regular presence over lifetime - Of >700 different serotypes (O,H,K), most are harmless
most subspecies remain with you for most of your life
- Pathogenic strains are assigned to ‘pathotypes’ based on the type of disease they cause and the virulance factor they harbour
- when refering to E.coli we differetiate via their pathotypes
Pathotype vs serotype?
Pathotype and serotype are both terms used to classify microorganisms, but they refer to different aspects of microbial characteristics:
Pathotype:
Definition: Refers to a classification based on the disease-causing ability or the pathogenic traits of a microorganism. Pathotypes are strains or groups of organisms that cause similar types of infections or exhibit similar virulence factors.
Focus: Pathogenic behavior or the ability to cause disease.
Example: Different strains of Escherichia coli can be classified into pathotypes based on the diseases they cause, such as:
Enteropathogenic E. coli (EPEC): Causes diarrhea, especially in infants.
Enterohemorrhagic E. coli (EHEC): Causes severe intestinal infections and can lead to kidney damage.
Serotype:
Definition: A classification based on the antigenic properties of an organism, particularly the recognition of different surface structures (such as proteins, polysaccharides, or lipopolysaccharides) by antibodies.
Focus: The immune response and antigenic variation, i.e., how the immune system recognizes different strains.
Example: Different serotypes of bacteria or viruses are distinguished by their surface antigens, such as:
Salmonella enterica has over 2,500 serotypes (e.g., Salmonella Typhi, Salmonella Enteritidis).
Dengue virus has four major serotypes (DENV-1, DENV-2, DENV-3, DENV-4).
Summary:
Pathotype refers to the pathogenic behavior and ability to cause disease.
Serotype refers to the antigenic properties recognized by the immune system.
Both can be used to differentiate between strains of the same species, but they focus on different aspects—disease-causing traits vs. antigenic characteristics.
“Serotype refers to the antigenic properties recognized by the immune system” means that different serotypes of a microorganism (such as a bacterium or virus) are classified based on their surface antigens, which are the molecules that the immune system detects and responds to.
E. coli disease pathotypes- intestinal infections (inPEC)
all cause gastrointestinal disease
- Enteroaggregative
E. coli-(EAEC)
-Enterotoxigenic
E. coli (ETEC)
-Enteroinvasive
E. coli (EIEC)
(Attaching and effacing
E. coli)-
no treatment for them
-Shiga toxin-producing
E. coli (STEC) or
Enterohaemorrhagic
E. coli (EHEC)
-Enteropathogenic
E. coli (EPEC) (Attaching and effacing
E. coli)
E. coli disease pathotypes- extra-intestinal infections
(ExPEC)
uropathogenic
E. coli (UPEC)- (cause UTIs)
meningitis-associated
E. coli (MNEC)- (cause Meningitis)
septicaemic E. coli (SEPEC)
Urinary tract infections (UTIs)
- 50% of women get a UTI in their lifetime
- 75% of UTIs are caused by uropathogenic E. coli
(UPEC)
-women are more likely to get infected due to shorter urethras - UTI infection: 14x more common in
females (shorter urethra) - In the pre-antibiotic era, 15% of UTI
cases were fatal - Types of UTI include:
asymptomatic bacteriuria (1% normally, 20% elderly)
cystitis (bladder infection)
pyelonephritis (upper ureter infection, kidney infection)
Mechanism of UPEC infection
Intestine of healthy
individuals contain UPEC
bacteria are good at sticking to the surface of the bladder and form biofilms or even go subcellular and reproduce
Periurethral contamination
with UPEC can occur after
a bowel movement or
during sexual intercourse
UPEC produce P-fimbriae which
recognises a blood
group antigen (Dgalactose-D-galactose)
found in 99% of people so is likely to infect people when they have the opportunity as they are likely to bind to the common receptor
E. coli colonisation loci in the human host - small intestine loci
Small intestine - EPEC | ETEC | DAEC | EAEC AIEC in Crohn’s disease (distal part ileum)
Epec and Ehec and how they are special in regard to the receptor tir
LEE is a pathogenicity island
tir is a Type 3 secreted bacterial protein that gets put into the host cell, it then localises to the host cell membrane and clusters there; this results in pedestal formation
proteins are injected into the host cell to aid with colonisation
