The human body is a favorable niche for bacteria as it provides all necessary requirements for their growth such as food, warmth, and moisture.
To obtain nutrition, bacteria invade the host cells by the means of virulence-associated factors and the process of pathogenesis.
Not all bacteria cause diseases, some are commensal in nature with the human host, for an instance, guts microflora.
Usually, opportunistic or virulent pathogens harm the host during pathogenesis.
Therefore, the disease is caused by the combined effect of damage caused during bacterial pathogenesis & the immune response against infection.
There are several virulence factors that are expressed such as toxins, which are major causes of damage.
Bacterial components trigger an immune response which leads to the production of acute-phase proteins such as TNF- α, IL-1 or IL-6.
Some E.g., Virulence associated factors, are cytotoxic proteins, degradative enzymes, superantigens, and toxins – Endotoxins & Exotoxins.
Mechanisms involved in Bacterial Pathogenesis
Initially, bacteria invade the host, if the abundance of the pathogen is high or host immunity is compromised–, it surpasses the host immune system.
Other than that, virulence factors and effective mechanisms help them to escape host immunity.
The steps of bacterial pathogenesis are as follows :
Steps of Bacterial Pathogenesis at a glance
Sources or Reservoir of the Bacterial Pathogens
In this case, reservoirs are the habitat or niche before or after infection where bacterial pathogens sustain and multiply.
The most common one is animal, environment or carrier, a human host.
Entry in the Human Host
Transmission of pathogens is carried in several ways:
it may be by direct contact for e.g., coughing/sneezing
Indirect contact for e.g., through the soil, food, or water
Skin is the first line of defense of the innate immunity, trauma, or wound that leads to a breach by the pathogen.
There are several other routes of entry which are as follows:
Bacterial Port of Entry (Adapted from book Medical Microbiology -Murray)
Attachment and Colonization by Bacterial Pathogen on the Human Host
After, the transmission of the bacterial pathogen into the suitable host, it initiates the adherence & colonization to the host cells/tissues.
Colonization states that the establishment of the bacterial pathogen on the particular site into the host cells or tissues for further growth and multiplication.
There are several obstacles for a pathogen to colonize in a host, it depends on how much the pathogen is able to compete with the microflora for nutrients.
Some bacteria are equipped with certain specialized components such as pili and fimbriae which facilitate their colonization.
a.) E.Coli Fimbriae            b.) Vibrios adhering the epithelial cells       c.) Candida albicans fimbriae
Additionally, capsule and adhesins are some virulence factors that also help in adherence & colonization.
E.g., of Bacterial Adherence (Adapted from book Medical Microbiology- Murray)
Invasion into the Host Tissues
After attachments, the bacterial pathogen invades the mucous membranes, which is mediated by lytic substances production.
Lytic substances disrupt the membrane.
Degrade molecular integrity of the cell i.e., protein-carbohydrate complexes.
Generally, pathogens enter the cell through passive mechanisms i.e., cuts, wounds, etc.
Bacterial pathogen invades deeper host tissues and continues disseminating, it may also enter the circulatory systems which can cause bacteremia, or the presence of bacterial toxins including bacterial cells can lead to septicemia.
Some virulence factors help in the bacterial dissemination process:
Coagulase – Clot the fibrinogens (present in plasma), helps to invade phagocytosis. Found in Staphylococcus aureus.
Hemolysins- Lyses erythrocytes (by which iron is readily available for pathogen), found in Streptococci, Staphylococcus, E.Coli
Growth and Multiplication of the pathogenic bacteria in the host
For successful growth & multiplication, bacterial pathogens need to find suitable environments (e.g., pH, nutrients, temperature) inside the host.
The host’s body has different areas with variable specific conditions, the most suitable site harbors the bacterial pathogens and allows their growth & multiplication.
Some bacteria grow in blood plasma, few within the host cells (facultative intracellular pathogens) for e.g., Brucella abortis, grow & multiplies in macrophages, neutrophils cells, these can be cultured in the lab, whereas obligate intracellular pathogens, like rickettsia, can’t be cultured without the host
Leaving the Host
To complete the disease cycle, bacterial pathogens need to escape the host successfully for infecting the new host.
It is employed by passive escape mechanisms such as urine, feces, saliva, droplets, or desquamating cells.
Nice.