Microbiology



L6: B. Clostridium botulinum

C. botulinum causes botulism, which occurs in several clinical forms. Botulism is caused by the action of a neurotoxin that is one of the most potent poisons known. It causes a flaccid paralysis.

Pathogenesis:

There are several types of botulinum toxin, designated A through G, but human disease is almost always caused by types A, B, or E. The botulinum serotypes and tetanus toxin constitute a homologous set of proteins whose neurotoxicity arises from proteolytic cleavage of specific synaptic vesicle peptides, causing subsequent failure of neurotransmission. In contrast to tetanus toxin, which causes constant contraction, botulinum toxins affect peripheral cholinergic synapses by blocking the neuromuscular junction and inhibiting release of the neurotransmitter, acetylcholine, preventing contraction and causing flaccid paralysis.

Clinical significance:

1. Infant botulism: or a cause of floppy baby syndrome. C. botulinum colonizes the large bowel of infants 3 to 24 weeks of age; the toxin produced is slowly absorbed. Constipation, feeding problems, lethargy, and poor muscle tone are common early signs. The condition is possibly a cause of sudden infant death syndrome, but recovery is the usual outcome, following symptomatic treatment that may be prolonged.

1. Wound botulism: A rare form of botulism occurs when a wound becomes contaminated with the organism, and toxin is absorbed from that site.

Laboratory identification:

The organism can be cultured and identified by standard anaerobic.

Treatment and prevention: Antitoxin, which neutralizes unbound botulinum toxin,

C. Clostridium tetani

The introduction of tetani spores into even small wounds via contaminated soil is probably a common occurrence. But a combination of the extreme O2 sensitivity of vegetative C. tetani and widespread immunization against its exotoxin, make the resulting disease, tetanus, rare in developed countries. Growth of C. tetani is completely local, but it produces a powerful neurotoxin that is transported to the central nervous system, where it causes spastic paralysis.

Pathogenesis:

Tetanus toxin, called tetanospasmin, is an extremely potent toxin. It is transported from an infected locus by retrograde neuronal flow or blood. The heavy fragment (B) mediates binding to neurons and cell penetration of the light fragment (A). The A fragment blocks neurotransmitter release at inhibitory synapses, and thus causes severe, prolonged muscle spasms. The A fragment has been shown to be a protease; it cleaves a small synaptic vesicle protein (synaptobrevin) and abolishes the flow of inhibitory neurotransmitters.

Clinical significance:

Tetanus has an incubation period varying from four days to several weeks. A shorter period is usually associated with more severe disease and wounds closer to the brain. Tetanus presents as a spastic paralysis, in which muscle spasms often first involve the site of infection. In the early stages of the disease, the jaw muscles are affected, so that the mouth cannot open (trismus‌). Gradually, other voluntary muscles become involved, and any external stimulus (for example, noise or bright light) precipitates a painful spasm, and sometimes convulsions. Death, which occurs in fifteen to sixty percent of cases, is usually the result of paralysis of chest muscles leading to respiratory failure.

Laboratory identification: is rod with terminal spore (drum stick), and characteristic swarming growth on anaerobic blood agar.

Treatment: Treatment with human hyperimmune globulin (tetanus immune globulin). The organism is sensitive to penicillin.

Prevention: Active immunization with tetanus toxoid prevents tetanus. It is usually administered to children as a triple vaccine with diphtheria toxoid and pertussis antigens (DPT).

L6 Eneterobacteriacea & Gastrointestinal Gram-negative Rods

Enterobacteriaceae is a large family of Gram-negative bacteria found primarily in colon of human and animals many as part of the normal flora.

General properties of members

1. found in the gastrointestinal (GI), facultative anaerobes, non spore forming, some spp. Has capsulate, some motile.

2. catalase positive, oxidase negative except Vibrio and Pseudomonas.

3. Reduce nitrates to nitrite. Major spp and related disease in following table.

Pathogenesis

They contain lipopolysaccharide (LPS) in their cell wall, which is both antigenic and a potential virulence factor (endotoxin). In addition several exotoxins are produce by E. coli and V. cholerae, these exotoxins are called enterotoxines that activate adenylate cyclase within the cells of the small intestine, causing diarrhea.

Antigenic structure: Three surface antigens:

1. cell wall antigen or somatic Ag (O Ag).

2. the flagellar antigen (H Ag).

3. the capsular or antigen or (K Ag) polysaccharide Ag in encapsulated bacteria.

Diagnosis of Enterobacteriaceae

-culture: Blood agar, Macconkey, eosine- methylen blue (EMB), detection of lactose fermentation on macconkey agar; lactose fermenter such as E. coli and Klebsiella forms pink colonies, while non lactose fermenter such as Shigella, Salmonella, Proteus, Enterobacter, Providencia, Morganella, Serratia and forms pale or colorless colonies. The selective effect of the media in suppressing Gram+ bacteria. Proteus and S. typhi produce H2S from sulfur-containing amino acids.

Enterobacter and Klebsiella have capsule.

-Serological test depending on the presence or absence of anti-H or anti-K or anti-O antibodies.

- molecular techniques by using PCR specific for selected bacterial genes.

|Major pathogens |Representative Diseases |

|Escherichia coli |Urinary tract infection, travelers diarrhea, neonatal meningitis |

|Shigella |Dysentery diarrhea |

|Salmonella, Citrobacter | Typhoid fever, Enterocolitis |

|Klebsiella |Pneumonia, Urinary tract infection, colonize hospitalized patients |

|Enterobacter |Pneumonia, Urinary tract infection colonize hospitalized patients |

|Proteus, Providencia, Morganella | Urinary tract infection ( nosocomial UTIs) |

|Serratia |Pneumonia, Urinary tract infection |

|Helicobacter |gastric ulcers |

|Campylobacter |food poisoning |

|Yersinia |Plague, Enterocolitis, mesenteric adenitis |

1. E. coli

• Diseases : Urinary tract infection, travelers diarrhea, neonatal meningitis, and sepsis. Nosocomial (hospital-acquired) infections: These include sepsis/bacteremia, endotoxic shock, and pneumonia. Habitat: human colon, vagina, urethra. From the urethra, it ascends and causes UTI acquired during birth neonatal meningitis and by fecal –oral route in diarrhea.

pathogenesis

1.Endotoxin in cell wall cause septic shock.

2.Also enterotoxin (heat labile toxin LT) which stimulates adenylate cyclase by ADP-ribosylation resulting in the increasing in cyclic AMP(cAMP) that causes out flow of chloride ions and water resulting in diarrhea.

3. enterotoxin (heat stable ST)causes dirrrhea by stimulating guanylate cyclase.

4.vero toxin (Shiga- like toxin) is an enterotoxin, it causes bloody diarrhea and hemolytic uremic syndrome associated with eating undercooked meat. Verotoxin inhibits the proteins synthesis. Following table shows strains of E.coli and disease.

Predisposing factors to UTI in women include the include the proximately of the anus to the vagina and urethra, as well as a short urethra. This leads to colonization of the urethra and vagina by the fecal flora. Colonization of vagina leads to neonatal meningitis acquired during birth.

Virulence factors: pili for attachment, and capsule that suppress phagocytosis.

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Prevention & Treatment

• Eating cooked meat and drinking boiled water, Doxycycline may be prevent traveler diarrhea. Maintenance of fluid and electrolyte balance is of primary importance in treatment.

• Prevention of UTI involves limiting the frequency and duration of urinary catheters.

• Prevention of sepsis involves removing or switching sites of I.V. catheters.

• There is no vaccine against E. coli infection.

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