The family Enterobacteriaceae
Tribe genus includedEscherichieae: Escherichia, Shigella
Salmonelleae Salmonella
Klebsielleae Klebsiella, Enterobacter
Proteeae Proteus
Opportunistic pathogens:
Genus Klebsiella (Friedlander bacilli)Members of this genus are found in the intestinal tract of human and animals or free living.
These MO are associated with a wide variety of opportunstics and nosocomial infections particularly pneumonia, wound infection and UTI.
The absence of motility distinguishs klebsiella from other members of enterobacteriaceae.
Kleb. Consist of several spp.:
Klebsiella pneumonia
Subspp ozaenae
Subspp rhinoscleromatis
K.oxytoca.
K. pneumoniae is the most common isolated spp. and has distinct feature of possessing a polysaccharide capsule which offer the mo protection against phagocytosis and antimicrobial absorption thus contributing to virulence. The capsule is responsible for moist mucoid colonies.
K pneumoniae causes pneumonia and pleuritis and a frequent cause of lower respiratory tract in immune compromised. It causes 3-7% of all nosocomial infections and has the ability to harbor antibiotic resistance plasmids.
Other extrapulmonary infections are enteritis, meningitis, UTI and septicemia.
K oxytoca is identical to K pneumoniae except for its production of indole.
LABORATORY DIAGNOSIS
Gram negative bacilli.
Culture:
MacConkey's agar: lactose fermenter large mucoid colonies.
EMB agar bluish black colonies with green metallic sheen
TSI: A/A+ gas no H2S
Biochemical tests:
IMViC - - + +
Urease +ve
Motility non motile
API
Genus Enterobacter
They are widely distributed in nature as water and are normal commensal enteric flora. It may causes opportunistic infections as UTI, respiratory tract infection, wound infection septicemia and meningitis. Enterobacter composed of 16 spp. The clinically significant spp include E. cloacae and E aerogenes.
Enterobacter has been documented as pathogen in neonatal meningitis and bacterimia, and it is isolated from brain abscesses.
LABORATORY DIAGNOSIS
Gram negative bacilli.
Culture
MacConkey's agar: lactose fermenter
EMB: blue black with metallic sheen.
TSI: A/A +gas no H2S
Biochemical reactions
IMViC: - - + +
Urease: -
Motility +ve.
API
Serratia
The genus composed 8 spp most important is Serratia marcescens. Serratia is an important opportunistic pathogen associated pneumonia, septicemia and bacteremia. Serratia are resistance to many antimicrobial agents. S. marcescens often produce red to pink pigment specially when incubated at room temperature.
LABORATORY DIAGNOSIS:
Gram negative bacilli.
Culture:
MacConkey's agar: lactose fermenter.
EMB: blue black colonies with metallic sheen.
TSI: K/A no H2S.
Biochemical tests:
IMViC: - - + +
Urease: -ve
Motility: +ve
API
Proteus
The genus proteus belongs to the tribe Proteeae. They are normal intestinal flora. They have a characteristic swarming motility which is a wave like spreading of the MO across the entire surface of the agar.
The most important spp. P. mirabilis and P. vulgaris.
P mirabilis causes UTI and wound infection. P. vulgaris also causes wound infection in immune-compromised and patients on antibiotic. UTI could be upper or lower.
LABORATORY DIAGNOSIS:
P mirabilis, P. vulgaris, are easily recognized in LAB by the characteristic swarming phenomena on non selective culture media (SBA). Also produce distinct oder burnt chocolate. Both spp hydrolyze urea making the urine of patient with UTI with proteus more alkaline promoting kidney stone formation by precipitation of Mg and Calcium. P mirabilis is differentiated from P. vulgaris by indole (mirabilis ve, vulgaris +ve).
P. vulgaris is sucrose fermenter and give K/acid in TSI.
All strains of P. mirabilis are sensitive to ampicillin and cephalosporin whereas P. vulgaris is resistant.
MO TSI Indole MR VP Ure Citrate motility
P vulgaris K/A gasH2S + + - + v + swarming
P mirabilis K/A gas H2S - + - + v +swarming
Gram negative
Culture:characteristic sweet odor
Nutrient and blood agar a characteristic swarming
MacConkey's agar: NLF colorless
EMB: colorless.
TSI: K/ acid + H2S
Biochemical tests
IMViC: + + - v P.vulgaris
-+ - v P. mirabilis
Urease +ve
Motility +ve
API.
Primary intestinal pathogens:
Salmonella (transmitted from animal, human carrier)
Shigella (primary human pathogen)
Salmonella
Salmonella is a major cause of bacterial enteric illness. Human infections are most commonly caused by ingestion of food, water or milk contaminated or due to contaminated poultry products.
Salmonella spp. are members of the family Enterobacteriaceae. They are gram negative, facultatively anaerobic rods. Salmonella spp. are classified into serovars (serotypes) based on the lipopolysaccharide (O), flagellar protein (H), and sometimes the capsular (Vi) antigens. There are more than 2500 known serovars. Within a serovar, there may be strains that differ in virulence.
Salomenlla enterica has 6 subsp.
S.enterica subsp. enterica
S. enteric subsp salamae
S.enterica subsp arizonae
S.enterica subsp diarizonae
S.enterica subsp huotenae
S.enterica subsp indica.
Virulence factors:
Enterotoxin significant virulence factor.
Fimbria in adherence.
Traverse intestinal mucosa.
Vi Ag (resist intracellular killing by phagocyte)
Antigenic Structures:
They have the somatic O Ag and flagellar H Ag in primary structure used in serological grouping of salmonella. Some salmonella posses K surface Ag and Vi Ag.
Vi Ag it is surface polysaccharide capsule it block the O Ag in serological typing but may be removed by heating.
Transmission
Salmonella spp. are mainly transmitted by the fecal-oral route. People are often infected when they eat contaminated foods of animal origin. Eggs are a major source of Salmonella in some countries.
Clinical infections
Four clinical types of infection
Acute gastroenteritis ( infective food poisoning):
Diagnostic Tests
As in animals, salmonellosis can be confirmed by isolating the organisms from feces or, in cases of disseminated disease, from the bone marrow or blood. In humans, bone marrow cultures are considered to be most sensitive. When using blood and stool, multiple samples may be tested.
Its the most frequent manifestation ranging from mild to severe diarrheoa accompanied by low grade fever. And varying degrees of nausea and vomiting. Symptoms appear 8-36 hr after the consumption of contaminated food. It is self limiting disease, no complications.
Enteric fever:
In the early period(1-2wk) there will be fever, constipation during which blood cultures are positive and stool culture negative, followed by the second diarrhoeic phase during which stool culture is positive and the blood culture become negative.
The clinical features of enteric fever include the prolonged fever, bacteremia, involvement of the RES (liver, spleen, mesentery, BM) and dissemination to multiple organs.
Typhoid is caused by Salmonella serotype Typhi, other enteric fever include paratyphoid caused by Salmonella Cholerasuis and S. Paratyphi. Paratyphoid is similar to typhoid but less severe and has less fatality rate.
IP is 9-14 days when the ingested mo reach the intestine it penetrates the intestinal wall and gain entrance to lymphatic's and reach blood stream then spread to the liver, spleen, BM were they are engulfed by phagocytes and multiply intra-cellularly and released to the blood stream for the second time. The mo isolated from the blood (1st week). During the 2nd and 3rd week of the disease the mo reach the gall bladder and peyer's patches the stool culture is positive and the gall bladder is focus for carrier.
Bacteremia:
Most common causal species: S. Choleraesuis, S Typhi and S. Paratyphi.
Symptoms: like sepsis caused by other gram-negative bacteria. It occurs without major GI symptom characterized by high spiking fever and positive blood culture. The risk of metastatic complication more severe than the bacterimia itself. 10% of patients may have localized suppurative infections, e.g., osteomyelitis, endocarditis, arthritis, etc.
High risk population: pediatric and geriatric patients; AIDS patients.
Carrier state:
In which persons with previous infection may continue to excrete the MO in their feces for up to one year following remission. The individual (1-5%) harbors the mo in the gall bladder. They excrete the mo continuously and intermittently they are important source of infection to susceptible persons.
DIAGNOSIS OF TYPHOID:
Salmonella infections can be confirmed by isolating the organisms from feces or, in cases of disseminated disease, from the bone marrow or blood. In humans, bone marrow cultures are considered to be most sensitive. When using blood and stool, multiple samples may be tested.
Gram negative bacilli
Culture:
BM culture 90% sensitive.
Blood culture positive in the 1st week.
Stool culture positive in the 2nd &3rd week.
Urine culture in the 2nd week,
In enterocolitis stool culture is positive.
On macConkey's agar NLF
EMB agar colorless
SS agar colorless colonies with black center due to H2S production
TSI : K/A +gas+H2S
Motility : positive
Biochemical tests
IMViC: - + - +
Urease: -ve
API test
Serological diagnosis: Widel test (the antigenic variation of S. Typhi. Many of the surface antigens of the other member of Enterobacteriaceae are shared and induce antibodies that are cross-reactive). Widal test antiO rise in the 8th -9th day, antiH rise in the 11th -12th day. 2ME for detection of IgG is useful.
The Widal test measures the specific antibody titre of the patient's serum to typhoid antigens by haemagglutination
PCR (highly sensitive and specific but no validated PCR test in common use)
Treatment
Enteric fever and bacteremia require antibiotic treatment: chloramphenicol, ampicillin, trimethoprim-sulfamethoxazole. Surgical drainage of metastatic abscesses may be required.
Salmonella enterocolitis needs only supportive therapy (antibiotic treatment may prolong the symptoms and excretion of the salmonellae). Drugs to control hypermotility of the gut should be avoided because it is easy to transform a trivial gastroenteritis into a life-threatening bacteremia by paralyzing the bowel.
Chronic carriers of S. Typhi may be cured by antibiotics alone or combined with cholecystectomy.
Shigella
Is a HYPERLINK "https://en.wikipedia.org/wiki/Gram-negative" \o "Gram-negative" gram negative, HYPERLINK "https://en.wikipedia.org/wiki/Facultative_anaerobic_organism" \o "Facultative anaerobic organism" facultative anaerobic, HYPERLINK "https://en.wikipedia.org/wiki/Endospore" \o "Endospore" non-spore forming, non-motile, rod-shaped HYPERLINK "https://en.wikipedia.org/wiki/Bacteria" \o "Bacteria" bacteriaclosely related to HYPERLINK "https://en.wikipedia.org/wiki/Salmonella" \o "Salmonella" Salmonella.
Shigella spp. are non lactose fermenter and tend to be biochemically inert.
Humans are the only host and the infection by ingestion. Outbreaks are most common in summer.
Antigenic structure:
The shigella spp are divided to 4 groups according to O Ag:
Dysenteriae group A (15 serotypes)( 1 produce shiga toxin).
Flexiniri group B (8 serotype).
Doydii group C (19 serotype).
Sonnie group D. (1 serotype).
The most severe form of shigellosis is caused by S. dysenteriae serotype 1.
Symptoms of disease
The clinical symptoms of shigellosis range from mild diarrhoea to severe dysentery (Dysentery is characterized by the presence of blood and mucous and pus in the stool) and the disease occur in epidemic dimension. The severity depends on the Shigella serotype causing infection, dose, the immunity and age of the host.
The incubation period is 17 days (usually 3 days) and symptoms typically last for 12 weeks. Initial symptoms include watery diarrhoea, fever and fatigue.
All Shigella spp. can cause acute bloody diarrhoea.
For most Shigella serotypes illness is generally self-limiting and fatality is very rare.
Virulence and infectivity
Once ingested, Shigella spp. must survive the acidic environment of the stomach and invade the epithelial cells of the colon to enable infection.
Shigella spp. multiplys inside the colonic epithelial cells, and spread to adjacent cells, leading to the death of the infected cells. The colon becomes inflamed and ulcerated and the dead mucoid cells are shed, resulting in the bloody mucoid diarrhoea often characteristic of Shigella infection.
S. dysenteriae serotype 1 strains produce the potent Shiga toxin which has cytotoxic, enterotoxic and neurotoxic effects.
Mode of transmission Shigella
Shigella spp. are transmitted by the faecal-oral route by either person-to-person contact, or consumption of contaminated food or water.
LABORATORY DIAGNOSIS
The diagnosis of HYPERLINK "https://en.wikipedia.org/wiki/Shigellosis" \o "Shigellosis" shigellosisis made by isolating the organism from diarrheal fecal sample cultures.
1- Culture
MacConkey's agar: NLF
SS agar colorless colonies
EMB agar colorless colonies
TSI : K/A no gas no H2S
2-overall biochemically inert.
urea hydrolysis negative.
Prevention and treatment:
Hand washing before handling food and thoroughly cooking all food before eating decreases the risk of getting shigellosis. Severe dysentery can be treated with ampicillin, TMP-SMX, or fluoroquinolones, such as ciprofloxacin, and of course rehydration. Antibiotics are usually avoided in mild cases because someShigellaspecies are resistant to antibiotics, and their use may make the bacteria even more resistant. Antidiarrheal agents may worsen the sickness, and should be avoided.