Microbiology Introduction

Question 1

Vertical transmission

Answer 1

from mother to child before, during or shortly after birth.

Question 2

fomite

Answer 2

An object that transmits an infection

Question 3

Horizontal transmission

Answer 3

From one person to another

Question 4

Zoonoses

Answer 4

infections acquired from vertebrates. They can occur by direct contact or in animal products

Question 5

vector

Answer 5

an arthropod that transmits an infection

Question 6

Adhesion to Host

Answer 6

Large parasites (e.g. worms) adhere by mechanical means, e.g. biting mouth parts. •

Microscopic organisms bind to host through receptor-ligand interactions. Ligands are normal host structures. Binding to a cell surface is the first step in entering host cell for intracellular pathogens

Question 7

Intoxication

Answer 7

Due to ingestion of a toxin that bacteria have produced in food. None of the microbe need be ingested or invasive in order to be intoxicated.

Question 8

Surface infection

Answer 8

Infection of the skin or mucosal surfaces

Question 9

Invasive infection

Answer 9

Extend into normally sterile spaces in the body.

Routes of spread of pathogens in the body include along the axons of nerves, within the blood or lymphatics, and by direct extension within tissues.

Question 10

antigenic variation

Answer 10

When a species of pathogen exists with different surface antigens (and expresses only some or one at a time) or when individuals within that species can change their surface antigens over time.

An effective immune response against one antigenic type does not prevent subsequent or prolonged infection with another antigenic type of that pathogen.

Question 11

Opportunistic pathogens

Answer 11

Cause infections in hosts with compromised defenses. Most opportunistic pathogens are environmental organisms that have not evolved to evade the immune response.

Question 12

virulence

Answer 12

The ability of a pathogen to cause damage

Question 13

Ways by which a pathogen may damage the host

Answer 13

• Killing host cells by intracellular microbes. This usually happens after the microbe replicates and lyses the host cell.
• Secreted toxins
• Effector molecules injected into host cells
• The host immune system
• Transformation of host cells
• Mechanical impairment of host organs
• Malnutrition

Question 14

The presence of such organisms at sites where they are part of the microbiota. . .

Answer 14

does NOT indicate an infection

Question 15

Most bacteria can devide every ___.

Answer 15

Most bacteria can devide every 20-30 minutes.

At least, during the logarithmic growth phase

Question 16

Role of peptidoglycan in bacterial cell walls

Answer 16

There is a high concentration of solutes within the bacterial cell, so water tends to move into the bacterial cell when they are in a hypotonic environment. This could lyse (pop) the bacteria. Bacteria must protect themselves from this osmotic stress.

Question 17

Mycobacterium

Answer 17

Has a thick waxy layer surrounding the cell instead of a conventional layer of peptidoglycan.

This includes Mycobacterium tuberculosis, the cause of tuberculosis. Mycobacteria are the only “acid-fast” bacteria, meaning they retain stains even when decolorized with strong acids. This is due to the thick waxy coating. Another characteristic of mycobacteria is that they replicate more slowly than most other bacteria.

Question 18

The Gram Stain

Answer 18

Question 19

Outer Membrane of Gram-Negative Bacteria

Answer 19

Have second membrane surrounding the cell, in addition to the cytoplasmic membrane. The outer membrane encircles the peptidoglycan. This gives rise to a periplasmic space.

The outer membrane contains a unique molecule called lipopolysaccharide or LPS.

Usually contains porins to facilitate diffusion of imporant molecules. Loss of porin expression is an important mechanism of development of antibiotic resistance, as antibiotics utilize porins to enter the cell.

Question 20

LPS

Answer 20

Question 21

Porins

Answer 21

Trimeric proteins that form a pore in the outer membrane. The inside of the pore is lined with charged amino acids, which facilitates small, charged molecules moving across the outer membrane. Charged molecules ~700 Daltons, or about the size of a tripeptide, can cross this membrane via porins.

Most antibiotics act on Gram negative bacteria in the periplasmic space or in the cytosol. Loss of porin expression is an important mechanism of development of antibiotic resistance.

Question 22

How hosts get around bacterial capsules

Answer 22

Antibodies!

But a few kinds of bacteria produce capsules that are identical to host molecules, and the body will not produce antibodies against such capsules. This may be considered another mechanism of immune evasion.

Question 23

How capsules enable immune evasion

Answer 23

First, capsules are charged, making them difficult for phagocytes to engulf. Second, some capsules downregulate activation of complement, reducing opsonization of bacteria.

Question 24

mechanisms of horizontal gene transfer in bacteria

Answer 24

conjugation, transformation and transduction.

Question 25

“Competent” bacteria

Answer 25

Capable of taking up DNA via transformation

Question 26

Conjugation

Answer 26

Question 27

temperate phages

Answer 27

Can take one of two pathways of growth in bacteria:

Lysogenic growth

Lytic growth

Question 28

Types of transposable elements

Answer 28

insertion sequence: Carries only the genetic information necessary for its own reproduction

transposon: Carries additional extraneous information

Question 29

C. difficile

Answer 29

Clostridium are anaerobic spore-forming Gram-positive rods responsible for several unrelated diseases with various clinical manifestations. C. difficile is harbored in the large intestine of a small percentage of healthy humans, where it tends to remain in low numbers. Most commonly colonizes children <1 y.o. and residents of nursing homes. The mode of transmission is via the spore form.

Question 30

C. difficile-mediated disease

Answer 30

Exposure to antibiotics establishes susceptibility to C. difficile infection. The most common antimicrobial drugs associated with C. difficile diarrhea are cephalosporins, ampicillin, and clindamycin. During or after antibiotic administration, spores germinate and the C. difficile grows in large numbers, producing its toxins. • When toxin production achieves a critical level in the large bowel, illness begins.

Results in diarrhea, fever and abdominal pain. Severe cases result in “pseudomembranous colitis” in which dead white blood cells and debris form a whitish yellow pseudomembrane on the surface of the colon.

Question 31

C. difficile-derived toxins

Answer 31

The organism produces two toxins (toxin A and toxin B) in the intestinal lumen, and the toxins damage the epithelial lining of the bowel wall.

These toxins dysregulate the cytoskeletal structure of the intestinal epithelial cells, which results in the breakdown of tight junctions between epithelial cells and resulting loss of epithelial integrity. There is a strong host inflammatory response as well.

Question 32

Answer 32

Pseudomembranous Colitis

Question 33

Diagnosis of C. difficile infection

Answer 33

Detection of the toxin in stool is highly specific but insensitive for disease.

Polymerase chain reaction (PCR) is highly sensitive for detection of C. difficile disease but it cannot distinguish between colonization and disease.

Detection of a non-toxin protein produced by C. difficile, called glutamate dehydrogenase or GDH, is sometimes used as a screening test for C. difficile, but confirmation of toxin production is needed to make a firm diagnosis after screening.

Question 34

C. difficile infection treatment and prevention

Answer 34

Oral antibiotics (metronidazole or vancomycin) can be used to successfully treat the overgrowth of C. difficile in the intestine.

Prevention: The antibiotics most commonly associated with the disease should not be used on susceptible populations of patients (e.g., those in nursing homes) if other antibiotics are available. Proper education in the methods of transmission of the disease, hand hygiene and appropriate use of gloves and gowns can reduce the spread of the organism.

Question 35

What determines gram positivity vs negativity?

Answer 35

Thickness of peptidoglycan wall!

NOT the membrane!

Question 36

Penicillin vs Ampicillin

Answer 36

Ampicillin is just penicillin with an extra amide.

Ampicillin’s extra group gives it an extra charge and makes it appear more like a tripeptide, making it easier to be picked up by porins.