Microbial Pathogenesis and Host Interactions: A Comprehensive Overview

Posted on Sep 20, 2024 in Biology

Microbial Pathogenesis and Host Interactions

Microbiota and Infection

Microbiota usually protects against infections, directly by limiting nutrients, inhibiting virulence gene expression and killing of incoming species. Indirectly, it alters host innate and adaptive immune response.

The outcome of infection is dependent on agent factors such as strain, gene content and expression. It is also dependent on the host’s genotype, age, history and immunity.

Types of Pathogens

Primary Pathogens

Primary pathogens can cause disease in immunocompetent hosts and can be obligate pathogens.

Mycobacterium tuberculosis – human specific obligate, infects 25% of the world population. Found in sterile deep tissue or within macrophages. Symptomatic disease occurs in 10% of infections.
Neisseria gonorrhoeae – Symptomatic disease is dependent on the infection site, with 90% occurrence in the penis and 20% in the vagina. Found in privileged sites such as submucosal areas and within neutrophils.

Opportunistic Pathogens

Opportunistic pathogens are non-obligate and only cause disease under specific conditions.

Pseudomonas aeruginosa – found in soil and water, low prevalence commensal, resistant to antibiotics, rarely causes disease in healthy people. Risk increases in burns, cystic fibrosis, and ICU patients.
Staphylococcus epidermidis – found on 80% of humans, can cause localised tissue inflammation or severe systemic disease. Risk increases in immunocompromised individuals or when there are breaches in the skin.

Host Immune Response and Pathogenesis

The host immune response is finely tuned, but an overly strong immune response leads to host damage and signs of disease.

Pathogenesis involves the pathogen gaining access to privileged host sites, replicating, persisting at those sites and interacting with the host to cause damage.

Virulence Factors

True virulence factors – cause host damage (e.g., toxins), facilitate colonisation (e.g., adhesin/pili), or help the pathogen avoid the immune system (e.g., capsule and LPS).
Accessory virulence factors – involved in nutrient acquisition, secretion of virulence factors, and regulated expression of virulence factors. These factors can also be used for non-virulence functions.

Experimental Models for Studying Pathogenesis

Animal Models

Animal models should display the same disease signs, similar tissue distribution, and be acquired via the same route as in humans. This is rarely actually achieved.

Species-specific differences – S. typhi does not infect normal mice, while S. typhimurium causes typhoid-like disease in mice but is non-systemic in humans.
Humanized mouse models – using human stem cells offer a middle ground.

Cell Lines

Cell lines reduce the use of animals and are better defined, but do not show complete disease, lack extracellular matrix, or other cell types.

Identifying Virulence Genes

Biochemical Approaches

Identify a protein and determine its mode of action.
Show that the protein’s action has a direct virulence effect.

Molecular Approaches

Identify a gene whose mutation affects virulence.
Defined for the model in which it is tested.

Koch’s Postulates

The gene is always found in strains with a particular virulence phenotype.
The gene is expressed in the host.
Mutation of that gene abolishes the virulence phenotype.
Reintroduction of the gene restores virulence.

Mutagenesis Techniques

Random Mutagenesis

Chemicals, radiation, transposons inactivate genes randomly.
Difficult to find which mutant has a change to the gene of interest.

Directed Mutagenesis

Makes a change only to the specific gene of interest.
Homologous recombination uses the natural recombination process in all cells to replace a gene with a copy that has been altered in vitro.
Can be insertion or deletion.

Other Techniques

Targetron mutagenesis – modified group II intron to insert into the gene of interest.
CRISPR – bacterial anti-bacteriophage defense system for making chromosomal mutants.

Gene Complementation

Find genes surrounding the gene of interest.
Sequence around the whole gene and parts of the surrounding genes.
This sequence undergoes homologous recombination to swap the gene of interest and the clone.
Clone the gene of interest into a plasmid that only replicates in E. coli.
Insert a new piece of DNA, usually an antibiotic resistance marker, into the gene of interest.
Transform the pathogen with a suicide plasmid (does not replicate) to create a strain with a directed mutant and antibiotic resistance.
Reintroduce the intact gene on a replicating plasmid to see if virulence has been restored (complement a mutant).

In Silico Methods

Sequencing methods make it easy and cheap.
Computer methods are well established.
Similarity to a known virulence factor does not prove the gene is involved in virulence.
Wet lab confirmation is needed: create a directed mutant, measure virulence, and perform complementation.

Transposon Mutagenesis

Transposons are transposable DNA elements that can integrate into a chromosome.
When integrating, they inactivate the gene.
Transposon insertion site sequencing (TIS) is a method of identifying essential genes under any growth condition.
TIS needs to make a large transposon library of up to 1 million mutants with insertions every 10-50 bp, assuming every gene has been mutated at least once.
Use high-throughput sequencing (HTS) to find the exact base of every transposon insertion site.
Genes without transposon insertion are assumed to be essential.

Genomic and Transcriptomic Approaches

Illumina Sequencing

Very high throughput.
Method relies on single base addition per cycle and fluorescent imaging.
Add base, image, uncap, then add the next base.

DNA Microarrays

Place every gene from an organism onto a glass slide.
Use hybridization to measure the transcriptional state of every gene.
Generally used to compare the transcriptional state between two different conditions, such as in vivo vs. in vitro and different media types.
Isolate bacterial RNA from the two conditions, label each with a different dye and mix, then read fluorescence.

Proteomics

Identify all proteins produced.
Comparative proteomics compares proteins produced in two conditions, telling you the amount.
Gel electrophoresis for separation by pH and size.
Most separations are performed by liquid chromatography.

From DNA to Proteins

DNA -(transcription by RNA polymerase)-> mRNA -(translation)-> Protein -(folding by chaperones)-> Post-translational modifications (protein processing/glycosylation/amino acid modifications) -> Final active proteins

Regulation of Virulence Genes

Genomic Changes

Gene amplification – a single copy of a gene is replaced by two or more copies of the same gene, commonly happens via recombination between repetitive sequences in two copies of a replicating chromosome. Two copies will usually produce more product than a single copy. Occurs from direct repeats being mistaken and pairing different DNA molecules.
Phase variation – alternation between two phenotypes, heritable, reversible, and occurs at high frequency. In a clonal population, most cells retain the expression phase of the parent, but a minority will change the expression phase. Observed in outer membrane proteins, capsule biosynthesis, LPS, pili, fimbriae, flagella, and DNA regulation proteins.
Genomic inversion – mediated by site-specific recombinase, occurs randomly and at high frequency in a population. The inverted segment contains a promoter that affects the expression of a neighboring gene. The change in the orientation of the promoter can be used as a switch to turn transcription on and off.
Slip strand mispairing – allows the high-frequency phase variation of certain proteins, which are often related to virulence. Genes contain short repetitive sequences near the 5′ end. Variation in the number of repeats results in a frame-shift mutation, turning the gene off. Different repetitive sequences of different lengths (not in multiples of 3) are used because they do not generate a frame-shift mutation, but rather a whole amino acid insertion. Variation in repeat number occurs through errors made during DNA replication.
Antigenic variation – alternation between the expression of different forms of antigenic proteins. High frequency allows bacteria to change the sequence of a gene. The new protein is no longer recognized by antibodies to the original protein. Recombination leads to an altered coding sequence and antigenic proteins with different sequences. Inversion can also be used for dual synchronized phase variation (e.g., Neisseria gonorrhoeae pilin expression). Different versions of the pilin gene are located in different places on the chromosome, but only one has a promoter. Recombination between unexpressed and expressed pilin genes alters the sequence of the expressed pilin gene. In Salmonella flagella, a repressor gene exists to block the promoter of H1 while H2 is transcribed.

Transcriptional Regulation

Operons – genes transcribed as part of a single transcript by a single promoter.
Regulons – genes in different locations but share promoter regions that respond to the same regulatory protein. Can have multiple operons.
Stimulons – a set of genes that respond to the same regulatory signal but not always the same protein.
Regulatory proteins – activators turn up transcription, and repressors turn down transcription. For example, LacI represses transcription unless bound to an inducer molecule. Activators help RNA polymerase bind.
Two-component signal transduction – sensor protein senses a signal, leading to phosphorylation of the sensor and a transducer protein. The phosphorylated transducer then induces a response regulator, which affects transcription.
Quorum sensing – detection of extracellular signaling molecules (cell-to-cell communication). Bacteria produce and secrete signaling molecules called autoinducers (AI). Upon AI concentration hitting a threshold, gene expression is altered. AI can be peptides or small cyclic molecules.
Alternative sigma factors – sigma factors are responsible for binding promoters. Different sigma factors bind to different promoters. Sigma factors control large regulons.

Post-Transcriptional Regulation

Altered translational efficiency – changes in the sequence or accessibility of the ribosome binding site, often associated with small RNA regulators.
Altered transcript stability – reduced opportunity for translation.
Post-translational modification – modification of the amino acid sequence, controls active protein production.
Efficiency of ribosome binding – the efficiency of ribosome binding to the ribosome binding site can be used to control the level of active virulence factor. The consensus sequence GGAGGA (reverse complement of 16S rRNA) is important for cholera toxin translation.
Post-translational modification – the protein translated from mRNA is not active. Most virulence proteins are exported and undergo some modification, usually proteolytic and sometimes glycosylation, and the addition of disulfide bonds.
Small regulatory RNA molecules – often antisense sequences, act by inhibiting the translation of mRNAs or increasing the rate of degradation of mRNA. For example, the agr system in S. aureus.

Cell Biology and Host-Pathogen Interactions

Functions of Membranes

Compartmentalization
Selectively permeable barrier
Transport system
Signal transduction
Intercellular interaction
Energy transduction

Membrane Components

Lipids – glycolipids, cholesterol, phospholipids
Proteins – peripheral, integral with a single transmembrane helix, peripheral covalently linked to lipid, and integral protein with multiple transmembrane helices
Carbohydrates – oligosaccharides and glycolipids covalently linked to lipids and proteins located on the outer face, highly variable and provides specificity

Extracellular Matrix (ECM)

Organized network of extracellular materials present beyond the immediate vicinity of the cell.
More than an inert packing material or nonspecific glue.
Plays a key regulatory role and determines the shape and activities of the cell.
Located beneath the basement membrane of epithelial cells.
Composed of macromolecules such as proteins (collagens, elastin, fibrillin) and carbohydrates (proteoglycans, hyaluronan, laminin) for structure, and adhesive glycoproteins (fibronectin, vitronectin, fibrinogen-fibrin).

Collagen

Most abundant protein in the human body.
Only present in the ECM and basement membrane.
Fibrous glycoprotein with a triple-helical structure.
Forms fibrils or networks and is key in providing structure.

Proteoglycans

Acidic, bind a large number of water molecules to form a porous gel.
Hyaluronan is the backbone of complexes made of proteoglycan molecules (polysaccharide with MW up to several million).
Provides strength and elasticity in tissues.
Component of artificial skin grafts.
Some pathogens (e.g., S. aureus and Streptococcus pyogenes) secrete hyaluronidase to break down hyaluronan.
Some pathogens secrete hyaluronan to bind to mammalian hyaluronan binding proteins.

Fibronectin

Has a cell-binding domain to bind to integrin.
Has domains for binding to heparin/fibrin, collagen, and fibrin.
Some bacteria express binding proteins that bind to fibronectin, which is an essential part of pathogenesis by helping colonization and penetration of host tissues.

Cytoskeleton

Regulates cell shape, movement, attachment, organelle localization, and cell division.
Consists of three types of protein filaments: intermediate filaments, microtubules, and actin filaments (microfilaments).

Microtubules

Alpha and beta tubulin bind GTP.
Rigid, dynamic, polarized.
Tracks for kinesin and dyneins, which move cargo.
Extend throughout the cytoplasm and function in cell shape, division, organelle movement, and are a component of cilia and flagella.

Actin Cytoskeleton

Actin filaments form a meshwork that extends throughout the cytosol and a dense web under the plasma membrane.
Composed of linear twisted actin protofilaments made of polymerized globular actin subunits.
Dynamic structure with functional polarity.
Association and dissociation of actin monomers are controlled by actin-binding molecules.
DAEColi is capable of actin accumulation and elongation of microvilli.

Polymerization of Actin

Nucleation – the most energetically unfavorable step. Three ATP-bound actins come together to form a nucleus. Unstable nuclei rapidly dissociate unless ATP is high. In vivo, nucleation is suppressed at all regions except those undergoing actin assembly. Actin monomer sequestering proteins probably limit nucleation by limiting the concentration of available free actin-ATP monomer.
Elongation – an indefinite series of monomer additions. Actin-ATP binds to the growing filament end, and ATP undergoes hydrolysis, and Pi is released while actin-ADP remains in the polymer.
Critical concentration – the critical concentration of a particular end of F-actin is the concentration of G-actin at which the association rate and dissociation rate are equal. Actin grows faster at the barbed (positive) end, which has a lower critical concentration, and vice versa for the minus end.
Actin monomer binding/sequestering proteins (e.g., thymosin beta 4) hinder filament assembly and block the exchange of ADP for ATP.
Actin polymerizing proteins (e.g., Arp2/3 and profilin) enhance filament assembly, increase ADP/ATP exchange, and bind to actin monomers.

Cell-Cell and Cell-ECM Contacts

The cytoskeleton is essential for maintaining cell-cell and cell-ECM contacts.
Desmosomes and hemidesmosomes (mediated by intermediate filaments) for cell-cell adhesion.
Adherens junctions and focal contacts for cell-matrix adhesion.
Both cell-cell and cell-ECM adhesions are associated with the actin cytoskeleton via a submembrane plaque composed of anchor proteins, signaling molecules, and adhesion molecules associated with the plaque.
Cadherins (adherens junctions) have calcium-binding sites.
Integrin αvβ3 is important for focal contacts (heterophilic interactions).

Cadherins

Mediate cell-cell adhesion, tissue formation, and integrity.
Calcium-dependent.
Transmit signals.
Belong to a multigene superfamily with specific cellular distributions.
E-cadherin for epithelial cells, N-cadherin for neural cells, and P-cadherin for placental cells.
Present extracellularly.
Listeria monocytogenes uses internalin A to bind to E-cadherin for entry.

Focal Contacts and Integrins

Integrins are integral membrane proteins with alpha and beta subunits non-covalently linked.
24 members known, with specific distributions.
The globular head binds ligands and contains a site to bind to the Arg-Gly-Asp (RGD) motif of fibronectin.
Calcium is required for ligand binding.
The cytoplasmic domain binds proteins and links integrin to the cytoskeleton.
Regulates actin cytoskeletal rearrangement.
Binding of ligand triggers intracellular signaling events.

Signaling Pathways

Extracellular receptor binds to a signal, leading to activation of intracellular signaling proteins, which then target metabolic, gene regulatory, or cytoskeletal proteins.
Stepwise process and modular, diverse, and multiple points of regulation.
Robust and responsive, easy to see activation or inactivation.

G Protein-Coupled Receptors (GPCRs)

7 transmembrane-spanning alpha helices.
Work with heterotrimeric GTP-binding proteins called G proteins.
Binding of a molecule leads to G protein activation, which produces second messengers through effector proteins.
G proteins are anchored to the membrane.
The alpha subunit binds GDP when inactive and GTP when active.
Also binds effector enzymes for second messenger systems.
Examples: cAMP, IP3, cGMP.

Receptors with No Enzymatic Activity

Transmit signals by interacting with protein kinases (e.g., cytokine receptors).
Usually associated with cell growth or immune function.

Enzyme-Linked Receptors

Receptor tyrosine kinases (RTKs) add phosphate groups to specific tyrosine residues.
The receptor monomer transverses the membrane only once and signals via a small GTPase (e.g., Ras).
Growth factor receptors are RTKs.

Small GTPases (Ras)

80 mammalian members.
Monomeric.
Molecular switches that relay signal transductions.
Cycle between an active form bound to GTP and an inactive form bound to GDP.
Activity is regulated by guanine exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine nucleotide dissociation inhibitors (GDIs).
Ras induces the formation of actin-rich structures, regulates cell cycle progression, gene expression, and membrane trafficking.

Adhesion Receptors

Integrins and cadherins for focal adhesions and cell-cell adhesions, respectively.
Signaling at focal adhesions: integrins bind to ECM molecules, leading to integrin clustering and activation of protein kinases. Src phosphorylates FAK. GRB2 and Sos bind, leading to Ras activation. Raf is activated, leading to activation of the MAP kinase cascade.
H. pylori expresses CagL on the surface of its type IV secretion system. CagL has an RGD motif to bind to integrin, which then activates the secretion system to pump the oncogenic protein CagA into host cells to alter host signaling pathways.
Adhesion receptors for signaling at adherens junctions: internalin A of Listeria monocytogenes binds to host E-cadherin.

Toll-Like Receptors (TLRs)

Transmembrane receptors involved in the dorsal-ventral patterning of embryos.
Mediate the immune response against fungal infection.
Share the Toll/IL-1R domain with mammalian IL-1R.
C3H/HeJ mice are known to have a defective response to LPS due to a natural tolerance from macrophages and B lymphocytes not responding to LPS. A spontaneous mutation in C3H mice was linked to LPS-defective responses. The mutation is a missense mutation in the Tlr4 gene.
TLR4 recognition: LPS is opsonized by LBP. The complex is recognized by CD14, which associates with the membrane by a glycolipid linkage.
Toll and TLR are conserved in invertebrates and vertebrates.
TLR4 recognizes LPS.
TLR3 recognizes dsRNA.
TLR5 recognizes flagellin.
TLR7/8 recognize ssRNA.
TLR9 recognizes CpG DNA.
TLR1/2 recognize diacyl lipopeptides.
TLR2/6 recognize triacyl lipopeptides.
TLR activation mediates signaling pathways leading to immune responses. PAMP detection by TLR leads to adapter molecule activation and thus into NF-κB or IFN responses.
TLRs are composed of leucine-rich repeats (LRRs) in the extracellular space and a Toll/IL-1 receptor (TIR) domain in the intracellular space. When bound to a ligand, TLRs undergo dimerization, leading to signaling.
Pathogen-associated molecular patterns (PAMPs): LTA, peptidoglycan, lipoprotein, LPS, flagellin, CpG.
TLR2 recognizes Gram-negative lipoproteins, lipoteichoic acid in Gram-positives, and some atypical LPS.
TLR9 recognizes unmethylated CpG dinucleotides, localized in the intracellular compartment on endosomes. Endocytosis of CpG leads to MyD88 recruitment.
TLR5 recognizes flagellin. It is exquisitely sensitive and conserved in animals, plants, and insects. Recognizes a conserved domain essential for motility. Produced in epithelial cells but expressed on the basolateral aspect of cells. Mediates pro-inflammatory responses and dendritic cell activation, and thus induction of the adaptive immune response. Does not recognize some flagellin types.

Nucleotide-Binding Oligomerization Domain (NOD)-Like Receptors (NLRs)

Conserved in plants, mammals, and fish.
Tripartite structure: N-terminal effector domain (variable), NOD (binds nucleotide, conserved), and C-terminal LRR domain (pathogen recognition).
NOD2 maintains tissue homeostasis in the gut and plays a role in Mycobacterium infection in mice.
NOD1 mediates innate immune responses to Gram-negative bacteria.
Both NOD1 and NOD2 recognize specific structures within the peptidoglycan layer. NOD1 recognizes GlcNAc and MurNAc, while NOD2 recognizes muramyl dipeptide. They both activate receptor-interacting serine/threonine protein kinase 2 (RIPK2), which leads to MAPK activation or IKK activation.
TIFA (TRAF-interacting forkhead-associated protein A) also responds to HBP release, leading to TRAF6 activation and NF-κB activation like that of NOD1. In the early stages of infection, it is NOD1-dependent, and in the late stage, it is ALPK1/TIFA-dependent.
Pathogens induce cellular stresses that can activate NOD1/2 signaling, such as ER stress, mitochondrial stress, and DNA damage, by detection of S1P.

Inflammasomes

Inflammasome activation is implicated in many major human diseases.
Promotes pyroptosis (inflammatory cell death), which releases intracellular contents and IL-1β, which increases inflammation.
Canonical inflammasomes are molecular scaffolds for caspase-1 activation in the cytosol, composed of NLR, apoptosis-associated speck-like protein (ASC), and caspase-1.
Interaction between self-oligomerizing PYD and CARD-containing proteins is key to inflammasome formation. PYD is a pyrin domain belonging to the death domain-fold family, which interacts with PYD and mediates NLRP-ASC interactions. CARD (caspase activation and recruitment domain) is in the same family and interacts with other CARDs and mediates ASC-caspase interactions.
Pyroptosis via cleavage of gasdermin D is mediated by inflammasomes.
NLRC4 inflammasome responds to flagellin, but bacteria evade by downregulating expression or changing residues used to activate NLRC4. NLRC4 interacts directly with caspase-1.
AIM2 requires interaction with ASC and caspase-1. Responds to bacterial DNA. Absent in melanoma 2.
Cytoplasmic Francisella novicida activates the AIM2 inflammasome upon escaping from the vacuole into the cytoplasm.
NLRP6 (Pyrin-NOD-LRR structure) is a negative regulator of NF-κB, a viral RNA sensor, and a metabolite sensor. Commensal bacteria activate signaling through TLR and metabolite signaling.
Goblet cell mucus secretion is IL-18 independent. TLR ligand is endocytosed, and ROS is produced by the goblet cell.
AMP production is IL-18 dependent.
Caspase-11 is a non-canonical inflammasome that activates gasdermin D and NLRP3. LPS and outer membrane vesicles activate the non-canonical inflammasome. Gram-negatives engage and promote caspase-1 activation. LPS or lipid A in the cytosol activates caspase-11.

Gut Microbiota and Health

Germ-free mice have many immune developmental deficiencies.
The human gut microbiota is composed of 4 main phyla: Firmicutes, Bacteroidetes, Actinobacteria, and Protobacteria.
The gut microbiota converts food to metabolites, such as conjugated bile acids to primary and secondary bile acids.
70% of the immune system is in the gut (GALT).
Gut homeostasis is dependent on a diverse microbiota. The microbiota interacts with the innate immune system but also helps develop the adaptive arm by inducing cytokines and phagocytic sampling.

Mucins

Specialized epithelial cells called goblet cells produce mucus.
The outer layer of mucus is occupied by anaerobic bacteria, and the inner layer is relatively sterile. This is the major barrier separating the cells from commensals.
Secretion is stimulated by bacteria.
Mucus glycans are cleaved by some commensals to liberate free sugars, suppress pathogens, and are used as a food source in some cases.
Secreted mucins are produced by Muc2, and cell-associated mucins are produced by Muc1.

Phagocytic Cells

Antigen-presenting cells process and exhibit small fragments of antigens and present them to T cells, bridging the innate and adaptive arms of the immune system.
These include macrophages, dendritic cells (sampling microbes from the lumen), and neutrophils.
They also make defensins (cationic peptides).

Cytokine Signaling

Cytokines are intercellular signaling molecules that induce a biological effect.
Both pro-inflammatory and anti-inflammatory cytokines exist.
IL-10, IL-4, and TGF-β are anti-inflammatory.

Beneficial Microbes

Bifidobacterium spp. – increase tight junctions, express serpins to inhibit neutrophil elastase and decrease the inflammatory response. Also increases goblet cell secretion of mucins by producing SCFAs (acetate producer).
Lactobacillus – secrete proteins for epithelial cell growth and inhibit apoptosis. S-layer protein A causes an increase in IL-10, binds directly to epithelial cells, and thus resists pathogen colonization.
Bacteroides fragilis – surface polysaccharide promotes homeostasis.
Clostridium – induce IL-10 and expand Treg cells to prevent inflammatory conditions. Produces butyrate.
Segmented filamentous bacteria (SFB) – attach to epithelial cells and protect against colonization. Pro-inflammatory and strengthens the gut barrier (both pro- and anti-inflammatory).
Faecalibacterium – supplies the SCFA butyrate.

Gut Dysbiosis

Dysregulation or perturbation of the gut microbiota decreases overall microbial functional diversity.
Reduction in Bacteroidetes and Firmicutes, with increases in Proteobacteria.
Leaves the host vulnerable to infections.
Linked to an increased inflammatory immune response.
Aging results in a reduction in diversity, thus reduced anti-inflammatory taxa and an increase in taxa with pathogenic and inflammatory species. Reduced mucin production.
Can be disturbed by antibiotic treatment, through direct microbiota depletion, modification of the host immune response, pathogen proliferation, and the emergence of opportunistic pathogens. Also, direct infection and inflammation result in microbiota depletion and aggravated histopathology.
C. difficile is opportunistic. Primary bile salts stimulate spore germination. Causative agents of gastrointestinal disease. Produces exotoxins A and B. Infection occurs post-antibiotic treatment due to unimpeded colonization as bile salt hydrolase-producing microbiota are killed off. Fecal microbiome transplantation is a proven way to transfer commensals from healthy donors.
Pathogens tend to kill or outgrow microbiota by expressing virulence factors.

Restoring Gut Health

Probiotics – ingesting live microbes to seed preferred microbiota.
Prebiotics – food to feed healthy microbiota.

Microbial Pathogenesis and Host Interactions: A Comprehensive Overview