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Medical Microbiology and Virology > 14 Entry, exit and transmission > Flashcards

14 Entry, exit and transmission Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

Pathogens have specific molecules that help bind to receptor molecules on host cells. Certain cells have certain receptors, which means that they are only susceptible to certain infections

CD4 molecule T cells - HIV
C3d receptor - EBV
Alpha-dystroglycan - M leprae

How does skin protect from infection?

A

Physical barrier
Dead layer
Fatty acids - pH 5.5

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2
Q

How do conjunctiva prevent infection?

What infections affect eye?

A

Lacrimal secretion - lysosyme
Blinking

Chlamydia - trachoma
Gonococci
HSV

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3
Q

Respiratory tract inhales 10 000 microbes per day, so needs good defenses.

What are the defenses?

How to microbes evade defenses?

A

Mucus + ciliary action
Cough
Macrophages

Attach firmly to surface of cell
Inhibit ciliary activity - Pertussis cytotoxin, pollution/ smoke
Ciliary destruction by virus - influenza, measles
Avoid phagocytosis - TB

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4
Q

How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name

  • Influenza A
  • Rhinovirus
A
  • Influenza A - haemagluttinin - sialyloigosaccharides

- Rhinovirus - capsid protein - ICAM-1

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5
Q

How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name

  • coxsackie A (common cold/ oropharyngeal vesicles)
  • parainfluenza type 1
  • RSV
A
  • coxsackie A - capsid protein - ICAM-1 or integrin
  • parainfluenza type 1 - envelope protein - sialoglycolipids
  • RSV - envelope protein - sialoglycolipids
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6
Q

How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name

  • mycoplasma pneumoniae
  • measles virus
A
  • mycoplasma pneumoniae - fuses with cell - neuraminic acid

- measles virus - haemagluttinin - CD46

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7
Q

How do following microbes attach to respiratory tract?
Microbial adhesion molecule/ receptor name

  • Haemophilus influenzae, strep pneumoniae, klebsiella pneumoniae
A
  • surface molecule - carbohydrate sequence in glycolipid
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8
Q

Intestinal transit time means that any ingested bacteria, are excreted in faeces. Infecting bacteria must attach to epithelium to prevent this. Some have specific attachments for small or large bowel - which is shown by symptoms they cause

Where and how do these microbes attach to GI tract

  • Poliovirus
  • Rota virus
A
  • Poliovirus - intestinal epithelium. Binds to Pvr (polio virus receptor)
  • Rota virus - intestinal epithelium. Viral capsid VP4 attach to host cell glycans
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9
Q

Where and how do these microbes attach to GI tract

  • Vibrio cholera
  • Escherichia coli (EPEC and EHEC)
A
  • Vibrio cholera - intestinal epithelium. Multivalent adhesion molecule 7 (MAM7)
  • Escherichia coli - intestinal epithelium. Bacteria inject Tir an effector into host membrane, acting as receptor for bacterial surface protein intimin
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10
Q

Where and how do these microbes attach to GI tract

  • Salmonella typhi (enteric fever)
  • Shigella (dysentery)
A
  • Salmonella typhi (enteric fever). Ileal epithelium. Binds using bacterial adhesins
  • Shieglla (dysentery). Colonic epithelium. Surface protein IscA acts as adhesin and interacts with host cells after activating type III secretion system trigerring its uptake into epithelial cells
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11
Q

Where and how do these microbes attach to GI tract

  • Giardia lamblia
  • Entamoeba histolytica
  • Ancylostoma duodenale (hookworm)
A
  • Giardia lamblia. Duodenal/ jejunal epithelium. Binds to mannose-6-phosphate on host cell, in addition to mechanical succker called ventral disc help bind to microvilli
  • Entamoeba histolytica. Colonic epihtelium. Lectin on surface of amoeba binds to host cell
  • Ancylostoma duodenale. Intestinal epithelium. Binds using large mouth (buccal capsule)
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12
Q

Some GI pathogens do not just attach, but attach and penetrate

Can you name examples?

A

Trichinella

Trichuris

Echinococcus

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13
Q

Bacteria motility can help them travel through protective mucus, and reach susceptible epithelium

Which bacteria have motility properties?

A

Vibrio cholerae

E. coli

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14
Q

Mucinase is an enzyme which helps break down mucus barrier.

Which bacteria produce this?

A

V cholerae

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15
Q

Bacteria generally prefer alkaline environment. But need to evade gastric acid.

Which microbes are resistant to acid?

A

H Pylori

Mycobacterium tuberculosis - intestinal TB

Enteroviruses - poliovirus, coxsachieviruses, echoviruses

HAV - infects liver, then shed in biliary tract into faeces

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16
Q

What are examples of organisms which can resist bile?

A 
Salmonella
Shigella
Enteroviruses
Enterococcus faecalis
E. coli
Proteus
Pseudomonas
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17
Q

Pathogens which are primarily anaerobic can thrive in GI tract.

What are examples of this?

A

Bacteroides fragilis

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18
Q

H pylori has special proteins to attach to gastric epithelium. It releases large amounts of urease, which creates a small surrounding area of ammonia in which they can survive. So treatment is antibiotics, as opposed to antacids

What are consequences of H pylori infection?

A

Gastritis
Gastric ulcers
MALT lymphoma

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19
Q

What lymphoid material is present in GI tract?

A

Peyer’s patches are localised collections of lymphoid tissues (MALT), lying just below epithelium surface. Contain “M cells” which take up foreign proteins and deliver them to underlying immune cells

20
Q

Urogenital tract is continuum, so once infected, organism can quickly spread.

What defences does vagina have?

A
  • Has glycogen on epithelium due to oestrogen. Lactobacilli colonise vagina, and metabolise glycogen to lactic acid - making pH about 5.0. Inhibits other bacterial growth except streptococci and diptherioids
  • for STI to infect they need to enter via injury to epithelium (genital wars, syphilis) or impaired defences (tampons, oestogren imbalance)
21
Q

Urinary tract has regular flushing as method of protection.

Exterior infection via urethra is almost always cause of infection.

Why are females at higher risk of UTIs?
14x more common in females

A

Shorter urethra 5cm (20cm men)

Urethra closer to anus

Sexual intercourse increases risk. Uncircumcised penis harbors bacteria

22
Q

What are natural defences of oropharnyx

A

Flushing action of saliva - 1L per day

IgA antibodies

Lysosyme

Normal flora

23
Q

What factors influence whether pathogen transmission will be successful?

A

Number of pathogens shed

Pathogens stability in environment - ability to resist drying, temperature etc. Some organisms produce spores (clostridium), or cysts (amoeba)

Number of pathogens required to infect fresh host - efficiency of infection

Ability to aid transmission - e.g causing coughing/ sneezing/ diarrhoea

24
Q

Once in external environemtn, organisms become dried. Those which are more stable on drying, are able to survive longer, and spread

What are examples of organisms which are more stable on drying?

A

Spread in air via dust/ dry droplets -

  • Tubercle bacilli
  • Staphylococci

Spread from soil -

  • Clostridial spores
  • Anthrax spores
  • Histoplasma spores

Spores can survive for years in soil

25
Q

What are routes human-human transmission of pathogens

A

Respiratory/ salivary - large droplets/ pathogens can travel 4m. Smaller diameter pathogens can float in air indefinitely, and travel far. Crowded areas such as schools/ nursing homes spread respiratory infections, and meningitis in military recruits.

Faecal-oral

Sexual

Blood

Vertical

Milk

26
Q

Why are respiratory pathogens more common in winter?

A

Colder temperature favours certain microbes growth

More close contact between humans

Heating - dry air impairs mucociliary clearance. More humid air favours growth of bacteria such as

27
Q

STIs require mucosal contact.

What are ways in which microbes make this possible?

A

Gonocci/ chlamydia - discharge

Treponema pallidum/ HSV - ulcers

28
Q

Which viruses have been detected in semen, and thought to be responsible for transmission?

A

HIV
HBV
CMV
Ebola

29
Q

Apart form organisms which commonly cause UTIs, what microbes are transmitted in urine?

A

Humans -

Schistosomes

Typhoid - bacteria in bladder scarred by schistosomiasis

Polyomaviruses -

  • JC virus PML
  • Merkel cell virus - Merkel cell cancer

CMV

Zoonoses -

Leptospirosis - infected rats and dogs excrete

Lassa fever - infected rodent excretes

30
Q

What are methods of transmission of zoonoses?

Virtually no human-human transmission, except for pneumonic plague and VHF.

Control aimed at controlling vectors/ animal infection

A

Arthropods - malaria, typhus

Vertebrae reservoir - brucellosis, rabies, Q fever, lassa fever, salmonellosis, ebola, MERS-CoV

Vector-vertebrate reservoir - plague, trypanosomiasis, yelllow fever

31
Q

What are common organisms transmitted via saliva?

A

HSV
CMV
EBV
Rabies

Pasteurella - in cats/ dogs, transmit via bites/ scratches
Streptobacillus moniliformis - rat saliva, cause rat bite fever

32
Q

What bacteria are commonly transmitted via the skin?

A

Staphylococci - impetigo, boils, neonatal sepsis
Streptococcus pyogenes - impetigo
Treponema pallidum - syphilis
Treponema pertenue - yaws

33
Q

Dermatophytes are transmitted via skin, and cause ringworm/ fungal infection

What parasites are transmitted by skin?

Which protozoa is transmitted by skin?

A

Scabies - eggs from burrow transmitted by hand

Leishmania tropica - cutaenous leishmaniasis, skin sores are infectious

34
Q

Milk is produced by a skin glad.

Which organisms are transmitted in human milk?

A

CMV
HIV
HTLV

35
Q

Which organisms are transmitted by animal milk?

Mostly cow milk

All organisms are destroyed by pasteurisation

A

Brucella (cow/ goat/ sheep)
Mycobacterium bovis
Coxiella burnetii Q fever
Campylobacter jejuni

Salmonella
Listeria
Staphylococci
Streptococcus pyogenes
Yersinia enterocolitica
36
Q

What is pasteurisation?

A

Process to sterilise milk

Heat treatment process - heating milk to high temperature to denature pathogens, then cooling

UHT (ultra high temperature) milk is treated at higher temperature, and immediately stored in packaging. Can last for months

37
Q

Which human infections are transmitted via placenta?

A

T - toxoplasma - stillbirth/ CNS disease
O - other - syphilis, listeria, VZV, parvovirus B19
R- rubella
C - CMV - abortion/ malformation

(H - HSV - not transplacentally, but during delivery)

HIV
HBV - via placenta, but usually from breast milk

38
Q

What does vertical transmission occur?

A

Via placenta, milk, or contact at birth

39
Q

Which perinatal infections occur in neonates, transmitted perinatally during skin contact?

A

HSV
Gonococcal
Chlamydia

40
Q

Many diseases are transmitted by arthropods. Once infection established, only ebola and plague are transmitted human- human

What are examples diseases causes by these classes of viruses?

Flaviviruses

Bunyaviruses

A

Flaviviruses

  • yellow fever
  • dengue
  • zika
  • west nile
  • japanese encephalitis

Bunyaviruses
- haemorrhagic fevers

41
Q

Many diseases are transmitted by arthropods.

What are examples diseases causes by these classes of bacteria?

Yersinia

Rickettsias

Spirochaetes

A

Yersinia

  • plague
  • tularaemia

Rickettsias

  • Q fever
  • spotted fevers
  • typhus
  • rickettsial pox

Spirochaetes

  • relapsing fever
  • lyme disease
42
Q

Many diseases are transmitted by arthropods.

What are examples diseases causes by these classes of protozoa?

Trypanosomes

Leishmania

A

Trypanosomes

  • sleeping sickness
  • chagas disease

Leishmania
- leishmaniasis

43
Q

Many diseases are transmitted by arthropods.

What are examples diseases causes by these classes of helminths?

Plasmodium

Nematodes

A

Plasmodium
- malaria

Nematodes

  • lymphatic filariasis
  • loiasis
  • onchocerciasis
44
Q

Domesticated animals such as cats and dogs can transmit multiple zooneses.

How can they transmit infection?

A

Bites/ scratches

Vectors carrying pathogen - ticks/ flies

Direct contact

Faeces

45
Q

Domesticated animals such as cats and dogs can transmit multiple zooneses.

What are examples of diseases which can be transmitted?

A

Bites/ scratches

  • rabies
  • pasteurella
  • Bartonella

Vectors carrying pathogen - ticks/ flies

  • ticks RMSF
  • sandflies leishmaniasis

Direct contact
- dermatophyte fungi

Faeces

  • toxocariasis
  • toxoplasma
  • hydatid
  • hookworm

Medical Microbiology and Virology (60 decks)

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