Microbial Interactions

Question 1

Symbiosis

Answer 1

an association of two or more different species of organisms

Question 2

Ectosymbiont

Answer 2

organism located on surface of another organism (usually larger)

Question 3

Endosymbiont

Answer 3

organism located within another organism

Question 4

Symbiont

Answer 4

physical contact between dissimilar organisms of similar size

Question 5

Consortium

Answer 5

interaction of one host with more than one symbiont

Question 6

Commensalism

Answer 6

Two organisms interact and only the commensal benefits, while the other is unaffected.

◦ Example 1: Nitrification

Question 7

Cooperation

Answer 7

Two organisms establish an interaction from which they both benefit, but this interaction is not obligatory; hence separation leaves both partners viable.

Examples:
1. Connection of carbon and sulphur cycles of Desulfovibro & Chromatium spp.

Question 8

Mutualism

Answer 8

Two organisms establish an very close interaction with reciprocal (mutual) benefit (on which they both depend.)

• Mutualism often involves syntrophy, an association where growth of either organism depends on or is
  improved by e.g. nutrients provided by the partner

Example 1: Plant+Rhizobium Example 2: Tube Worm+Bacteria

Question 9

Amensalism

Answer 9

Interaction in which one organism has
an adverse effect on another without benefiting.

Question 10

Competition

Answer 10

Occurs when two organisms try to use the same resource
◦ Two possible outcomes of competition
1. “Winner takes it all” = exclusion
2. Two organisms share a resource
 both survive at lower population levels

Question 11

Parasitism

Answer 11

An organism that lives in (endoparasite) or on (ectoparasite)
another (host) to its own advantage’ – to the disadvantage of the host

Question 12

Predation

Answer 12

organism attacks, kills and feeds on
prey organisms

Question 13

Formation of biofilms

Answer 13

1. Substratum preconditioning by ambient molecules
2. Cell deposition
3. Cell adsorption
4. Desorption
5. Cell-to-cell signalling and onset of exopolymer production (bacterial communication to signal synchronous switch to sessile life style)
6. Convective and diffusive transport of O2 and nutrients
7. Replication and growth
8. Secretion of polysaccharide matrix
9. Detachment, erosion, and sloughing (bacterial communication to signal synchronous production of release factors)

Question 14

Infectious Dose (ID50)

Answer 14

the number of pathogens that will infect 50% of an
experimental group of hosts in a specified time

Question 15

Lethal Dose (LD50)

Answer 15

the number of pathogens that will kill 50% of an
experimental group of hosts in a specified time

Question 16

Classification of infections

Answer 16

1. by causative agent
2. by bodily site of infection
3. by mode of transmission
4. by source

Question 17

The course of infectious disease

Answer 17

1. Incubation period
   ◦ period after pathogen entry, before signs and symptoms
2. Latency period
   ◦ some pathogens can lay dormant without causing
   symptoms after infection; but can cause disease upon specific triggers
3. Prodromal stage
   ◦ onset of signs and symptoms not clear enough for diagnosis
4. Period of illness
   ◦ disease is most severe, signs and symptoms
5. Convalescence
   ◦ signs and symptoms begin to disappear

Question 18

Pathogenicity Islands

Answer 18

Contain virulence genes
◦ Present in pathogens, absent in non-pathogenic relatives
◦ Deviate often in G+C content from core genome
◦ Often inserted adjacent to tRNA genes
◦ Frequent association with mobile genetic elements, i.e plasmids, and phages,
integrative and conjugative elements (ICE)
◦ Genetic instability (if functional mobility elements are present)
◦ Mosaic structure due to repeated gene loss and acquisition

Question 19

Endotoxins

Answer 19

Endogenous components of bacteria which have toxic effects on host

◦ Released predominantly passively, shedding
◦ Very heat-stable

1. LPS => lipid A portion very toxic => sensed by host receptors => inflammatory signalling -> Septic shock
2. Outer Membrane Vesicles (OMV) – vesicles budding
   from the bacterial surface -> contain LPS, proteins, lipids
   -> Can fuse with membranes of host or other bacteria
   -> Manipulation of recipient cell

Question 20

Exotoxins

Answer 20

Heat labile molecules, proteins which are produced for release by bacteria to manipulate their environment

◦ Require active export out of the bacteria
◦ Need to cross membranes, cell wall etc.
=> Bacteria evolved sophisticated protein secretion systems for protein export

Question 21

Type VII secretion system

Answer 21

• versatile weapon for competition and host manipulation
• found in mycobacteria and gram-positive
• substrates are transported as dimers
• c-terminal secretion signal

Question 22

AB Toxins

Answer 22

• A(Activity) subunit - responsible for toxic effect
• B(Binding) subunit - binds to specific receptor on target cell

Question 23

Motility Systems - flagella

Answer 23

Long surface appendages (propellers) driven by molecular motor

 Allow for example evasion of patrolling phagocytic immune cells

!Flagella are highly immunogenic! => loss of flagella can be advantageous for adaptation to human host

Question 24

Motility System - actin-based motility

Answer 24

Actin-polymerisation at one bacterial pole pushes bacteria forward

 Evasion of intracellular defences which aim to entrap the bacteria

 Can mediate cell-to-cell spread in epithelial layers

Question 25

Invasion of host cells (2 parts)

Answer 25

- zipper - Exploitation of host cell receptor binding inducing cytoskeletal rearrangement and uptake - trigger - Entry of virulence factors which induce cytoskeletal rearrangement and uptake

Question 26

Type III secretion systems

Answer 26

Found in Gram-negative bacteria ◦ Needle/syringe like – injection systems ◦ Effectors contain an N-terminal secretion signal ◦ Cargo is transported unfolded ◦ Direct injection in host cell, but also secretion into exterior possible

Question 27

Type IV secretion systems

Answer 27

Found in Gram-negative bacteria ◦ Diverse family of secretion systems  Prototype T4SSA from Agrobacterium tumefaciens  Prototype T4SSB Legionella pneumophila ◦ Related to bacterial DNA conjugation systems ◦ Often no clear needle complex ◦ Can transport DNA and unfolded proteins ◦ Effectors contain a C-terminal secretion signal

Question 28

Type VI secretion systems

Answer 28

T6SSs share similarity to reversed bacteriophage ◦ Act against bacteria and/or host cells ◦ Could facilitate human infection if used against the protective microbiome ◦ Can be coupled to expression of DNA uptake systems -> scavenging of foreign DNA ◦ No secretion signal defined yet

Question 29

Plague - "Black death"

Answer 29

Pathogen: Yersinia pestis Disease ◦ 3 forms: bubonic, pneumonic and septicemic plague transmission - vector-mediated (flea) - human to human - airborne If not treated, death in 50-70% of bubonic cases within 3-5 days. Pneumonic cases up to 100% death if not treated within 24h. - type III SS - T3SS effectors interfere with phagocytosis and immune signalling treatment - antibiotics

Question 30

Tuberculosis

Answer 30

transmission - human to human - airborne - low infective dose ID50 disease Respiratory infection, leading to destruction of the lung with expectoration of bloody sputum => progression from infection to disease very slow => latency can be years or decades 1/3 of global population are thought to be infected => activation and progression favored by immune suppression => rise of disease with HIV, most frequent cause of death for HIV patients

Question 31

Primary Infection Process of MTB

Answer 31

Pathogen: Mycobacterium tuberculosis MTB ◦ Primary infection process: 1. Survival in alveolar macrophages by evasion of phago-lysosomal degradation and/or by killing 2. Attraction of various immune cells which begin to encapsulate the infection focus – tubercle/ granuloma 3. Incoming macrophages and neutrophils phagocytose bacteria and dead cells containing bacteria, can become a new niche for replication 4. Immune activation of these cells increases their potential to kill MTB => Complex interactions between bacteria and immune cells control disease progression -> much still to be understood

Question 32

Secondary Infection Process of MTB

Answer 32

Infection focus develops in: A. Protective granuloma -> Clearance of MTB B. Homeostatic granuloma -> Latency/ Dormant MTB C. Transmissive granuloma -> Growth & release into lung Progression depending on innate and adaptive immune mechanisms Progression from B -> C can occur years after infection

Question 33

Leprosy

Answer 33

Pathogen: Mycobacterium leprae – 183000 new cases - 2023 infects and damages peripheral nerve and skin cells ◦ Transmission: Human-to-human, direct contact Long latency, 1-20 years, average 5 years ◦ Disease: Tuberculoid form: non-progressive, loss of sensation in affected area Lepromatous form: expansive, aggressive destruction of skin tissue loss of features, toes etc.; nodules covering body ◦ Treatment: Multi-drug therapy over several months

Question 34

Buruli Ulcer

Answer 34

Pathogen: Mycobacterium ulcerans ◦ Rare disease (~2000 cases globally, 2023) Transmission: Mosquito (evidence in Australia) ◦ Key virulence factor: ◦ Mycolactone: toxin; encoded on large virulence plasmid => interferes with correct production and delivery of proteins ◦ Treatment: Combinatorial antibiotic therapy

Question 35

Streptococcus pneumoniae

Answer 35

Gram-pos. bacteria, pairs or short chains of rounded bacteria ◦ Most frequent cause of typical pneumonia ◦ 30-50% of adult community-acquired pneumonia (CAP) ◦ Colonises the nasopharynx of 5 –60% of healthy persons (decreases with age) ◦ Transmission in nasal secretions from person to person ◦ Invasive disease more likely in the very young or old and people with underlying health conditions

Question 36

Streptococcus pneumoniae - key virulence factors

Answer 36

Capsule (CPS): prevents entrapment by mucus, evasion of opsonophagocytosis ◦ Multiple Proteases and Glycosidases (ZmpA, BgaA etc..), degradation of IgA1 and mucus ◦ Multiple adhesins: Ancillary pilus subunit RrgA etc.. for initial adherence and induction endocytosis ◦ Pneumolysin (Ply): toxin inhibits cilial beating, cytotoxic and pro- apoptotic for a wide variety of host cells, paracellular invasion

Question 37

Group A Streptococcus

Answer 37

Group A Strep (GAS) = Streptococcus pyogenes ◦ Rounded bacteria, often longer chains ◦ Carried asymptomatically, but can cause -> common illnesses such as pharyngitis, impetigo and scarlet fever -> life-threatening diseases such as sepsis, necrotizing fasciitis and toxic shock

Question 38

Legionella pneumophila

Answer 38

Gram-negative, rod-shaped bacteria * >65 Legionella spp. have been isolated; but L. pneumophila serogroup 1 strains cause >90% of clinical cases. * Severe, possibly fatal pneumonia: Legionnaires’ disease. * Most at risk are the elderly and persons with comprised immunity or respiratory malfunction. * Important cause of (4-14%) community and hospital-acquired pneumonia. * Incidence in the US and Europe on the rise in the past decade * Key virulence factor is a type IV secretion system

Question 39

Enteric Infection - Cholera

Answer 39

Pathogen: Vibrio cholerae ◦ Disease Acute diarrhea with extremely watery stools “rice water stool” ◦ Occurrence: Asia, Middle east, Africa 1.3 to 4.0 million cases, 21 000 to 143 000 death yearly (WHO) ◦ Intestinal colonisation: Adherence to intestinal mucosa ◦ Key virulence factors 1. Cholera toxin; AB-toxin, carried by phage => induces opening of ion channels -> efflux of water and ions 2. Toxin-coregulated pilus (TCP) – facilitates adhesion to mucosal surfaces

Question 40

E.Coli

Answer 40

- interact in diverse ways with the intestinal mucosa EPEC/STEC(EHEC) introduce their own receptor into host cells which interacts with the bacterial ligand “intimin” on bacterial surface => cause attaching and effacing lesions Key virulence factors Toxins: ◦ Heat-labile enterotoxin (LT), AB toxin ◦ Heat-stable enterotoxin (ST) ◦ Shiga-toxin (Stx) (AB toxin) Type III secretion systems & effectors: ◦ LEE-pathogenicity island, nle (effectors) ◦ pINV (Shigella) Adhesins: ◦ Afa/Dr fimbriae ◦ Bfp pili ◦ Colonisation factors (CF)

Question 41

Salmonella

Answer 41

Pathogen: Salmonella enterica ◦ Subspecies: enterica, salamae, arizonae, diarizonae, houtenae, and indica >2000 serovars => 99% of cases caused by Salmonella enterica enterica ◦ Non-Typoid Salmonella: Infect humans and various animal hosts ◦ Salmonella Typhi: human specific ◦ Disease: ◦ Non-Typoid Salmonella: Self-limiting diarrhoea ◦ Salmonella Typhi: Systemic infection, high fever, various effects on whole body, diarrhoea at later stage of disease, once spread to organs might cause fatal complications – septic shock etc. – app. 200000 death per year (world-wide) ◦ Transmission: Fecal-orally, food- or water-borne ◦ Salmonella Typhi maybe transmitted from human to human and shed for long time after last symptoms have passed

Question 42

Assessing bacterial virulence

Answer 42

- lethality - infection of model organism -> survival curves -> LD50 can be determined - cytotoxicity Various phenotypes can be assessed: * Cell morphology (rounding), cell detachment by microscopy * Release of cytosolic content vs. flux of membrane impermeable dyes * Assays for other markers of cell death (for example host enzymes involved) - growth in model organism - infection of model organism -> sacrifice and plating of bacteria -> growth curve

Question 43

Bioluminescence lux genes

Answer 43

Luciferase alone can be used if substrate (luciferin) is provided, e.g. by injection ◦ Luciferase-mediated light production is used as reporter for various applications e.g. promoter activity in bacteria, eukaryotes and many other organisms

Question 44

Visualisation of bacteria using fluorescent proteins

Answer 44

Bacteria can grow in association (extracellular) with or inside (intracellular) host cells * Exploitation or evasion of phagocytosis can be an important virulence mechanism => distinguishing and quantifying extra- and intracellular bacteria is key to understand the infection strategy Extracellular bacteria will appear red and green, intracellular green only