Case 3 - Healthcare-associated Infection

Question 1

Define reservoir

Answer 1

The reservoir of an infectious agent is the habitat in which the agent normally lives, grows, and multiplies. Reservoirs include humans, animals, and the environment. The reservoir may or may not be the source from which an agent is transferred to a host.

Question 2

What are asymptomatic/passive/healthy carriers?

Answer 2

These carriers are those who never experience symptoms despite being infected.

Question 3

What are incubatory carriers?

Answer 3

These are those carriers who can transmit the agent during the incubation period before clinical illness begins.

Question 4

What are convalescent carriers?

Answer 4

These are those carriers who have recovered from their illness but remain capable of transmitting to others.

Question 5

What are chronic carriers?

Answer 5

These are those carriers who continue to harbor a pathogen such as hepatitis B virus or Salmonella Typhi, the causative agent of typhoid fever, for months or even years after their initial infection.

Question 6

Define zoonosis

Answer 6

This refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans. Long recognized zoonotic diseases include brucellosis (cows and pigs), anthrax (sheep), plague (rodents), trichinellosis/trichinosis (swine), tularemia (rabbits), and rabies (bats, raccoons, dogs, and other mammals). Many newly recognized infectious diseases in humans, including HIV/AIDS, Ebola infection and SARS, are thought to have emerged from animal hosts, although those hosts have not yet been identified.

Question 7

Define the portal of exit

Answer 7

Portal of exit is the path by which a pathogen leaves its host. The portal of exit usually corresponds to the site where the pathogen is localized.

Question 8

Outline the modes of transmission

Answer 8

Direct: 
> Direct contact
> Droplet spread
Indirect:
> Airborne
> Vehicleborne
> Vectorborne (mechanical or biological)

Question 9

Define portal of entry

Answer 9

The portal of entry refers to the manner in which a pathogen enters a susceptible host. The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act. Often, infectious agents use the same portal to enter a new host that they used to exit the source host.

Question 10

Define a susceptible host

Answer 10

The final link in the chain of infection is a susceptible host. Susceptibility of a host depends on genetic or constitutional factors, specific immunity, and nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity. An individual’s genetic makeup may either increase or decrease susceptibility.

Question 11

What are the 5 moments of hand hygiene?

Answer 11

1) Before touching a patient
2) Before clean/aseptic procedure
3) After touching a patient
4) After touching patient surroundings
5) After body fluid exposure risk

Question 12

Outline the structure of bacterial cell walls

Answer 12

The cell wall of most bacteria consists of a lipid bilayer membrane and a mesh like peptidoglycan layer. Gram positive bacteria build a thick peptidoglycan sheath around a single membrane, while gram negative bacteria typically build a thin layer of peptidoglycan between two membranes. Water constantly enters bacterial cells by osmosis, building up pressure on the cell membrane. Peptidoglycan allows the cell to resist this pressure by providing structural support for the membrane.

Question 13

Outline the structure of peptidoglycan

Answer 13

At the molecular level, peptidoglycan is made up of small building blocks, each composed of 2 sugars connected to a short chain of amino acids, with a peptide bridge extending to the side. These sugars are assembled into chains, but then cross linked by the peptide bridges to form a tough peptidoglycan matrix. The enzyme known as Penicillin binding protein assists with peptidoglycan matrix assembling by creating the cross links between chains.

Question 14

Outline the mechanism of penicillin

Answer 14

Penicillin disrupts the creation of the bacterial cell wall, by preventing them from building their peptidoglycan layer, causing the bacteria to burst under pressure. Penicillin blocks the function of the penicillin binding protein enzyme by making a direct bond to a key serine amino acid in its active site. The active portion of penicillin is a beta-lactam ring, which is chemically reactive and opens up to form a bond to the active site serine. This inactivates the penicillin binding protein and prevents the proper formation of the peptidoglycan matrix.

Question 15

Where is the beta-lactam ring found

Answer 15

It is found in both natural and synthetic antibiotics that are structurally similar to penicillin, such as: amoxicillin, methicillin and ampicillin. Beta-lactam antibiotics are effective against many different types of bacterial infections, but bacterial communities have began to adapt to survive.

Question 16

What resistance mechanisms have bacteria developed against antibiotics?

Answer 16

1) MRSA, a strain of Staphylococcus aureus, expresses Penicillin binding protein 2a, which has an altered active site that doesn’t bind beta-lactam antibiotics.
2) Some bacteria express special beta-lactamase enzymes which bind to beta-lactam antibiotics and break the beta-lactam ring, making the antibiotic ineffective.

Question 17

What drugs have been developed to fight bacterial resistance?

Answer 17

1) Some drugs block beta-lactamases directly, leaving the antibiotic free to inhibit Penicillin binding proteins.
2) Othee new beta-lactam drugs (e.g. CEFEPIME and MEROPENEM), are either not recognised or not broken down by some beta-lactamases.
Unfortunately, in the face of the overuse or misuse of antibiotics, bacteria continue to develop ways to resist the action of new drugs.

Question 18

What are plasmids?

Answer 18

These are the small circular pieces of DNA on which are encoded many of the genes crucial for antibiotic resistance. Plasmids can be transferred from one population of bacteria to another, via bacteriophages, and then from generation to generation.

Question 19

What is the NDM1 gene

Answer 19

This is a particularly dangerous gene found on plasmids that allow bacteria to build the New Delhi metallo-beta-lactamase enzyme. These types of enzymes can break down almost all known beta-lactam drugs, posing a major global health threat.