Gram Negative Bacilli PDF

Gram negative bacilli B Y A LT A Y I B Z A K A R I A MSC IN MEDICAL MICROBIOLOGY  ESCHERICHIA COLI  It is also the most common aerobe to be harbored in the gut of humans and animals  Virulence factors of E.coli  Virulence factors of E. coli may be grouped into surface ant...

Gram negative bacilli B Y A LT A Y I B Z A K A R I A MSC IN MEDICAL MICROBIOLOGY  ESCHERICHIA COLI  It is also the most common aerobe to be harbored in the gut of humans and animals  Virulence factors of E.coli  Virulence factors of E. coli may be grouped into surface antigens and toxins 1. Somatic or O antigen: It is the lipopolysaccharide ( LPS) antigen. It is heat-stable. Responsible for endotoxic activity; It protects the bacteria from phagocytosis and bactericidal effect of complement. 2. Flagellar or H antigen.  It is heat labile  Presence of H antigen (flagella) makes the bacteria motile  hence contributing to their virulence. 3. Capsular or K antigen  It may also contribute to virulence by inhibiting phagocytosis.  It is expressed by only few strains of E.coli e.g. those causing neonatal meningitis, pyelonephritis and septicemia. 4. Fimbrial antigen (pilus) is the organ of adhesion, helps in attachment and colonization.  It is expressed by a few strains of E.coli. 2. Toxins :  The exotoxins secreted by E. coli are of several types: 1. Enterotoxins 2. Heat labile toxin 3. Heat stable toxin 4. Verocytotoxin. 5. Hemotysins 6. Cytotoxk necrotizing factor 1 (CN F1) and secreted autotransportertoxin (SAT). 7. Siderophores (i.e. aerobactin)-Helps in iron uptake. Epidemiology Escherichia coli accounts for more than 90% of the more than 7 million cases of cystitis and 250 000 of pyelonephritis estimated to occur in otherwise healthy individuals every year in the United States. Urinary tract infections are much more common in The reservoir for these infections is the patient’s own intestinal E.coli flora, which contaminate the perineal and urethral area. In individuals with urinary tract obstruction or instrumentation, environment sources assume some importance Pathogenesis : Relatively minor trauma or mechanical disruptions can allow bacteria colonizing the periurethral area brief access to the urinary bladder. Clinical Manifestations E.coli one of the most common pathogen encountered clinically and has been associated with various manifestations. 1. Urinary tract infection (UTI): It is caused by Uropathogenic E.coli (UPEC) 2. Diarrhea: It is caused by six types of diarrheagenic E.coli 1. Enteropathogenic E.coli (EPEC) 2. Enterotoxigenic E.coli (ETEC) 3. Enteroinvasive E. coli (EIEC) 4. Enterohemorrhagic E.coli (EHEC) 5. Enteroaggregasive E. coli (EAEC) 6. Diffusely adheren E. coli (DAEC) 3. Other infections: I. Abdominal infections: E. coli is the most common cause of body primary bacterial peritonitis (occurs spontaneously) and secondary bacterial peritonitis II. It also causes visceral abscesses, such as hepatic abscess. III. Pneumonia ( especially in hospitalized pacients vencilator-associated pneumonia) IV. Meningitis ( especially neonatal meningitis) V. Wound and soft tissue infection such as cellulitis and infection of ulcers and wounds, especially in diabetic foot VI. Osteomyelilis VII. Endovascular infection and bacteremia.  LABORATORY DIAGNOSIS  Sample collection: Depends on the site of infection- urine, stool, pus, wound ,swab etc  Direct smear: Gram-negative bacilli, and pus cells  Culture Media : Blood agar , MacConkey agar  Biochemical identification: Catalase positive and oxidase negative Nitrate is reduced to nitrite , indole(+), Citrate (-), Urease(-), TSI: A/A, gas(+),  Sugar fermentation test: Ferments most sugars  Antimicrobial susceptibility testing  TREATMENT  Acute uncomplicated UTIs are often treated empirically.  Because of widespread resistance to earlier agents like ampicillin, trimethoprim/sulfamethoxazole (TMP- SMX) or fluoroquinolones are used for this purpose. Selection of other antimicrobials must be guided by antimicrobial susceptibility testing of the patient’s isolate.  Treatment with TMP-SMX or fluoroquinolones reduces the duration of diarrhea in ETEC, EIEC, and EPEC infection.  SHIGELLA Shigella : it the most important agent of bacillary dysentery. Shigella differ from E.coli being nonmotile and not fermenting most sugars except mannitol. Classification Based on a combination of biochemical and serological characteristics, shigellae are classified into four species- S. dysenteriae, S.flexneri, S.boydii and S. sonnei which are also designated as serogroups A, B, C and D respectively Antigens and Serotyping of Shigella Based on somatic O polysaccharide antigen, Shigella species are further typed into four groups. 1. S. dysenteriae (group A) 2. S.flexneri (group B) 3. S. boydii (group C ) 4. S. sonnie (group D) Other Antigens of Shlgella 1. K antigen may be present in some serotypes 2. Fimbrial anigens may be found in some strains, especially in S. flexneri. 3. Flagellar H antigen is absent..  Pathogenesis  Shigella is one of the important causes of bacillary dysentery. 1. Mode of transmission: Infection occurs by ingestion through contaminated fingers (most common), food, and water or rarely flies.  It can also be transmitted sexually (homosexuals).  Entry via M cell: Bacilli enter the mucosa via M cells.  They cross the basolateral side of M cells to reach the submucosa where they are engulfed by macrophages.  Subsequently, the macrophages release bacilli.  Cytokines are released by infected intestinal epithelial cells, which attract increased numbers of inflammatory cells to the infected site, exacerbating inflammation, and leading to the acute colitis that characterizes shigellosis. Invasion: Once inside the submucosa, shigellae induce their own uptake into the adjacent epithelial cells. Invasion is determined by Ipa proteins and type Ill secretion system,They inturn help in uptake of the shigella into the host cells. Direct cell to cell -cell spread: Shigellae spread directly from one host cell to the other by inducing actin polymerisation of host cells, mediated by ksA proteins.  Exotoxins: 1. Shigella enterotoxin (ShETl and 2)  ShET1 is structurally similar to cholera toxin and is found essentially in S.flexneri 2a.  ShET2 is present in all S. flexneri isolates. It helps in iron uptake. 2. Shiga toxin: It is a cytotoxin, produced by S. dysenterioe type 1.  It is structurally and functionally similar to verocytotoxin of EHEC. It inhibits protein synthesis.  It enhances local vascular damage of intestine as well as internal organs, such as kidney and brain. 3. Endotoxin: It acts similar to any other gram-negative endotoxin and induces intestinal inflammation and ulcerations  Clinlcal Manifestations :  Shigellosis typically evolves through five phases: 1. Incubation period: it usually lasts for 1-4 days. 2. Initial phase : is characterized by watery diarrhea with fever, malaise, anorexia and vomiting. 3. Phase of dysentery: it is characterized by frequent passage of bloody mucopurulenr stools with increased tenesmus and abdominal cramps.  Phase of complication: it is commonly seen with children less than 5 years age.  Intestinal complications: perforations and rectal prolapse.  Metabolic complications, such as hypoglycemia, hyponatremia, and dehydration.  Bacreremia is rare and can lead to meningitis and pneumonia. Rarely, cases of vaginitis and keratoconjunctivis is have been reported. Epidemiology Risk factors for shigellosis include overcrowding, poor hygiene and children less than 5 years. It rends to occur as epidemics in developing countries such as Indian subcontinent and sub-Saharan Africa. LABORATORY DIAGNOSIS Specimen: Fresh stool Culture Enrichment broth such as- Selenite F broth, tetrathionate broth and gram-negative broth Highly selective medium, e.g. DCA, XLD agar and SS agar Biochemical reactions: Catalase positive and oxidase negative Antimicrobial susceptibility testing Treatment : Ciproffoxacin is the drug of choice. Alternative drugs which are effective are ceftriaxone, azithrormycin, and some fifth-- generation quinolones. Duration of treatment is about 3 days except for: S. dysenterioetype 1 infection- 5 days.  Infections in immunocompromized patients- 7- 10days. Oral rehydration solution (ORS) should be started for correction of dehydration and nutrition should be started as soon as possible after the completion of initial rehydration.