16 - Pathogenic Virus Families affecting Humans

Virus Families Pathogenic for Humans

Family Representative Viruses Type of RNA/DNA Lipid Envelope RNA Viruses Picornaviridae Poliovirus Coxsackievirus Echovirus Enterovirus Rhinovirus Hepatitis A virus (+) RNA No Caliciviridae Norwalk agent Hepatitis E virus (+) RNA No Togaviridae Rubella virus Eastern equine encephalitis virus Western equine encephalitis virus (+) RNA Yes Flaviviridae Yellow fever virus Dengue virus St. Louis encephalitis virus West Nile virus Hepatitis C virus Hepatitis G virus (+) RNA Yes Coronaviridae Coronavirusesa  (+) RNA Yes Rhabdoviridae Rabies virus Vesicular stomatitis virus (–) RNA Yes Filoviridae Marburg virus Ebola virus (–) RNA Yes Paramyxoviridae Parainfluenza virus Respiratory syncytial virus Newcastle disease virus Mumps virus Rubeola (measles) virus (–) RNA Yes Orthomyxoviridae Influenza A, B, and C viruses (–) RNA, 8 segments Yes Bunyaviridae Hantavirus California encephalitis virus Sandfly fever virus (–) RNA, 3 circular segments Yes Arenaviridae Lymphocytic choriomeningitis virus Lassa fever virus South American hemorrhagic fever virus (–) RNA, 2 circular segments Yes Reoviridae Rotavirus Reovirus Colorado tick fever virus ds RNA, 10–12 segments No Retroviridae Human T-lymphotropic virus types I and II Human immunodeficiency virus types 1 and 2 (+) RNA, 2 identical segments Yes DNA Viruses Hepadnaviridae Hepatitis B virus ds DNA with ss portions Yes Parvoviridae Parvovirus B19 ss DNA No Papovaviridae Human papillomaviruses JC virus BK virus ds DNA No Adenoviridae Human adenoviruses ds DNA No Herpesviridae Herpes simplex virus types 1 and 2b Varicella-zoster virusc Epstein-Barr virusd Cytomegaloviruse Human herpesvirus 6 Human herpesvirus 7 Kaposi's sarcoma–associated herpesvirusf ds DNA Yes Poxviridae Variola (smallpox) virus Orf virus Molluscum contagiosum virus ds DNA Yes

aIncluding the coronavirus causing severe acute respiratory syndrome (SARS). bAlso called human herpesvirus (HHV) 1 and 2, respectively. cAlso called HHV-3. dAlso called HHV-4. eAlso called HHV-5. fAlso called HHV-8. Abbreviations: ds, double-strand; ss, single-strand.

15 - Facultative anaerobes part 5 ( PASTEURELLACEAE )

PASTEURELLACEAE : 

• Pasteurella

o Pathogen of domestic animals (septacemia = cattle; fowl cholera = chickens)

o P. multocida transmitted to humans by dog / cat bites.

o Nutritionally fastidious; requires complex media for growth in the laboratory


• Haemophilus:

o Common inhabitants of upper respiratory tract mucous membrane, mouth, vagina, GI.

o Requires X factor (unknown component of hemoglobin) for synthesising parts of the cytochrome system; eg choclate agar (blood heated & gives brown color) supllemented with NAD or NADP (V factor).

o H. influenzae - flu like

o H. ducreyi -- veneral disease aka soft chancre

o H. suis-- hog influenza

14 - Facultative anaerobes part 4 ( VIBRIONACEAE )

VIBRIONACEAE : 

• Curved to comma shaped

• Polarly flagellated

• Facultative anaerobes

o Aerobic & therefore catalase & oxidase +ve (similar to aerobic Pseudomonads)
o Fermenters ie anaerobic

• Widespread distribution (fresh water, sea water) & human & animal intestinal tracts); not indicators of fecal pollution.

• 3 genera (Vibrio, Aeromonas, Photobacterium)

o Vibrio Cholera (aka Asiatic cholera)
 Two biotypes of serogroup O:1 known (classic cholera & Eltor [Mecca camp]); Eltor endemic in coastal regions eg India, Bangladesh & South America; USA non-O1 serogroup due to consumption of contaminated seafood.
 Transmitted thro' sewage contaminated water & food; rice water stools
 Multiplies in intestinal epithelial cells (but not invasive to other tissues) & produces an enterotoxin (binds irreversibly to epithelial cells stimulating the production of cyclic which changes cell permeability leading to the secretion of water & electrolytes into the lumen. 12 to 20 litres of water lost-- dehydration; replacement of electrolytes & water necessary is the best treatment; Tetracycline decreases disease period; vaccines provide short immunity but natural infections provide a longer duration (against enterotoxin & cells)
 Culture in alkaline medium (high pH decreases the growth of other bacteria)
 Vibrio parahemolyticus
 Habitat is salt water and requires > 2% NaCl for growth in laboratory media (halophilic) Gastroenteritis from contaminated shell fish, crabs; USA, Japan, Hawaii Large nos. within 24 hr & recovery follows in a few days.

o Aeromonas
 Mainly pathogens of fresh water fish, turtles & frogs.
 May be encountered as causative agents of human diseases.

o Photobacterium
 Interesting due to the property of bioluminescence (aka luminescent bacteria); ATP + electron transport chain + enzyme luciferase = luminescence
 Normal flora of some fish (flash light fish); behavioral activities (mating, protection)

13 - Facultative anaerobes part 3 ( ENTEROBACTERIACEAE )

• Escherichia :

o Most common inhabitants of intestinal tracts.

o Food & water presence indicates fecal contamination; indicator bacterium

o Endogenous spread (septacemia, urinary tract infections, meningitis) or food-water borne transmission (gastroenteritis, travellers diarrhea)

o Antigenic composition complex: 170, 57 H and several K antigens (epidemiological & specific serotypes associated with increased virulence (H157:O7)

o Septicimia: Focus of infection is urinary tract or intestine and spreads to bloodstream (higher incidence of death in immunocmpromised patients)

o Urinary tract infection: 80% = community aquired or hospital aquired. 10 specific serotypes usually involved (eg 01, 02, 016, 075) & origin is from gastrointestinal tract.

o Pathogenecity --serotypes more adhesive to uroepithelial cells than other serotypes

o Meningitis: E.coli & B streptococci cause 75%. Serotype K1

o Gastroenteritis: 4 groups; enterotoxigenic (ETEC) causes travellers & infant diarrhea, pathogenecity similar to V. cholera toxin ie stimulates hypersecretion of fluids & electrolytes in gastrointestinal tract , not serotype specific, produce heat labile & heat stable plasmid mediated enterotoxins, not serotype specific (NA probing, tissue culture or animal models & not serotyping for identification. Enteroinvasiveness (EIEC)



• Klebsiella : 

o Capsulated

o K. pneumoniae-- pneumonia (old or immunocompromised eg alcoholics); also causes septecimia in hospital pediatric wards



• Serratia : 

o S. marcesens- red pigmented colonies

o Causes hospital aquired infections (nosocomial) like Enerobacter

o Urinary & respiratory tract infection

o Isolated from sterile saline solution, catheters

  

• Proteus : 

o Extremely motile

o P. mirabilis-- urinary tract infection; produces urease & splits urea to CO2 and NH3 to increase pH & therefore facilitate renal stone formation & uroepithelium toxicity.

o Pili decreases phagocytosis & therefore decreases virulence unlike E. coli .



• Erwinia : 

o Plant pathogens (phytopathogens)

o Produce pectinase which hydrolyse pectin between plant cells & cause plants to rot

12 - Facultative anaerobes part 2 ( ENTEROBACTERIACEAE )

• Salmonella :

o 2000 serotypes based on H (flagella protein), O (outer membrane protein) & K (Capsular protein)

o Antibodies obtained commercially for serotyping using Kauffmann-White scheme.

o Some Salmonella serotypes are host specific (humans= S. typhi S. paratyphi; animals (S. cholerae-suis but cause severe disease if transmitted to humans) & some others not host specific (affect humans & animals)

o Food borne disease mainly thro' poultry, eggs & dairy products

o Exposure to Salmonella is frequent but requires a high dose of 1 to 100 million (eg improperly refrigerated foods; low dose for immunocompromised, very young and the elderly or with decreased gastric acidity

o Disease is due to microbial growth in body tisssue & not by ingestion of foods contaminated with toxins as a result of microbial growth

o Salmonella infections occurs in one of four forms:

 Gastoeneteritis (S. enteritis) most common form; eating contaminated foods; elevated temperatures, cramps & head ache. Symptoms disappear spontaneously in 2 to 7 days
 Septicimia--All Salmonella can cause bacterimia (S. cholera-suis, S. typhi & S. paratyphi)
 Enetric fever (S. typhi- typhoid fever & S. paratyphi- paratyphoid fever)-- Similar symptoms but paratyphoid fever milder; Transmission by consumption of contaminated foods --> multiplication in intestine (endothelial cells)--> invades lymphatic tissues to blood stream & kidneys (excreted in urine)
 Asymptomatic carriage-- Establishes in gall bladder (ressists bile & bile salts); continuous feedback into intestine; known as carrier state (2-5% typhoid patients become carriers & excrete 1 to 1000 million S. typhi /gm feces & also in urine); Carrier state is important in transmission of the disease. Mary Mallon (1901), aka Typhoid Mary, was the first carrier case to be detected; In 15 years infected 200 & jailed to prevent spred of typhoid fever. Disease has declined due to curative & preventative measures (heat killed injectable vaccines or the new live oral vacciine; same efficacy but later has less side effects


• Shigella : 

o Simple taxonomy (4 species & 32 serotypes) unlike Salmonella

o Found in humans, apes & monkey only unlike Salmonella

o Cause bacillary dysentry (shigellosis); S. sonnei (industrialized nations) & S. flexneri (developing nations); S. sonnei mild but S. dysenteriae (aka Shiga bacillus produces Shiga toxin) severe.

o Shigella enter endothelial cells (intestine) & multiply causing tissue damage; exotoxins stop host cell protein synthesis. Rarely (with the exception of S. dysenteriae) cause septicimia. 20 bowel motions a day.

o Transmission by fecal-oral route thro' contaminated hands & usually not by food-water

o Low dose required-- 200 Shigella cells can establish disease

11 - Facultative anaerobes part 1 ( ENTEROBACTERIACEAE )

• Produce acid & gas anaerobically
• Medically very important
• 3 families:
o ENTEROBACTERIACEAE (peritrichous)
o VIBRIONACEAE (polar)
o PASTEURELLACEAE (none)
________________________________________
 
ENTEROBACTERIACEAE : 

General comments - 
 
• Most important heterogenous collection of medically significant bacteria (27genera & 110 sp)
 
• Ubiquitous- soil, water, vegetation (saprophytes) & some normal flora of humans & animals but opportunistic (E.coli, Klebsiella pneumoniae, Proteus mirabilis) and some always pathogenic (Salmonella, Shigella & Yersinia pestis).
 
• Infections can originate from an animal reservoir (Salmonella), from a human carrier (S. typhi, Shigella) or by endogenous spread of the organism in a suspetible patient (E. coli)
 
• Readily culturable on non-selective media from sterile specimens (spinal fluid, tissues), selective media eosin methylene blue (EMB), MacConkey agar for contaminated specimens (sputum, feces), highly selective media (eg stool samples-- normal flora mask) & cold enrichment technique (stool sample + saline --> 4 C / 2 weeks; most enerobacters killed but not Yersinia enterocolitica (slow grower)
 
• Identification based on fermentation products (types & ratio) eg Butanediol (Enterobacter); mixed acids (E. coli; Salmonella & Shigella-- ratios of products important) , serological properties but cross-reactions limit usefulness [flagella (H), lipopolysaccaharide (O) & capsular (K); eg E.coli (170H, 56H & numerous K), biochemical propeties identification within 4 to 24 hrs; Capsules-- Klebsiella
 
• Treatment with antibiotics (E. coli, Proteus miabilis) but some highly ressistant & in some cases not recommended (S. enteriditis, prolongs carrier state). Antibiotic ressistant bacterial infections commonly hospital acquired (not community aquired)
 
• Pathogenecity : 
 
• Endotoxins (lipid A of LPS) released from cell lysis , flagella, pilli & fimbrae in adherence & attachment; capsules --weakly antigenic & protect against antibody mediated phagocytosis