Bacterial Pathogenesis: How Bacteria Invade Your Body

 To bacteria, the human body is like a playground, or a lush hotel, containing a variety of environmental niches that almost seem custom-made to suit their needs. We have got everything they need to grow and thrive in our nooks and crannies, from moisture and warmth, to food and protection. As we’ve evolved as a species, so have bacteria.

  Over time, bacteria have gained or lost genetic traits that allow them to adapt and better survive. Gaining or losing a trait may allow them to invade a particular environment better, survive longer in a particular niche, more effectively break down food, or, perhaps most importantly, evade detection by the immune system. Unfortunately, some of these adaptations can wreak havoc on our bodies.

  In many cases, bacteria might gain or develop enhanced virulence factors, which allow them to more effectively cause disease. Virulent bacteria grow and thrive at the expense of their host. For instance, some bacteria might release toxins that can travel through the blood, causing life-threatening disease. Others might be able to directly degrade our tissues or trigger aggressive cascades within our immune system.

In many cases, the symptoms that we experience are actually caused by an excessive inflammatory or immune system response triggered by the infection, and not necessarily the bacteria it self.

Assessing  Degree of Disease

  The degree of disease that bacteria can cause depends on a few things.
* importance of affected tissue or organ.
   You can imagine that an infection of the central nervous system would be extremely serious, while an infection of your left pinky toe might not be as life threatening.
* strain of bacteria and inoculum size
Another factor is the perticular strain of bacteria and how much there is of it, which is called the “inoculum size”. Some bacteria, like Shigella, which causes food poisoning, require a relatively tiny inoculum size, like around two hundred, to cause serious gastrointestinal distress, while others, like Salmonella, might require several orders of magnitude more inoculum, like a million or more, to cause a serious infection.
* States of Host
However, the particular host factors into this equation as well.
If you are immunocompromised, for instance, it might take much less Salmonella to make you sick.

Causes of Bacterial Destruction.

1. Entry
  Our bodies have natural defense mechanisms, such as skin, earwax, stomach acid, tears, and mucous membranes. Our skin prevents microorganisms from invading,our tears contain enzymes lysozyme that attack bacteria, our airways filter out harmful particles, and our mucous membranes are coated with secretions that fight off microorganisms.
  Despite our body’s best efforts, some of these microbes are able to bypass these defenses. For instance, bacteria such as Salmonella,Vibrio, Bacillus cereus, and Shigella can enter the body through ingestion. This could be from a picnic lunch left out in the sun for too long, or as the result of poor hand washing.
Other bacteria, such as Streptococcus, Mycobacterium,or Legionella enter through inhalation, perhaps after a sick person’s sneeze, or infected aerosol particles floating through the air.

Clostridium tetani, the causative agent of tetanus, enters through trauma or a wound.
  Other portals of entry include a mosquito bite, needles tick injuries, or sexual transmission.

2. Adhere and Colonize
  Bacteria have a vast arsenal of mechanisms to both adhere to surfaces within the body and colonize, which means to establish a microbial presence and multiply, once they have made contact.
For instance, prokaryotic cells have short, hair-like structures called fimbriae or pili that they use attach to various surfaces in nature. Some bacteria have adhesions on the tips of these pili that have specifically evolved to allow them to bind tightly to cells in your body. i.e. the pili of Neisseria gonorrhoeae bind specifically to oligosaccharide receptors on epithelial cells.

Other bacteria express adhesion proteins in a variety of ways. Another bacterial adaptation that promotes colonization is the formation of biofilms, which are collectives of one or more types of microorganisms.

Within a biofilm, bacteria form sticky webs of polysaccharides that bind bacterial cells together into a community, providing protection from antibiotics or host defenses.

Bacteria like Pseudomonas aeruginosa can sense when enough bacteria are present and trigger biofilm formation through a process called quorum sensing. Biofilms are particularly common on catheters,in dental plaque, or on implanted surgical devices such as pacemakers.

3. Specific ways bacteria cause damage.

For some bacteria, natural byproducts of their growth can cause tissue destruction. For instance, in your gut, anaerobic bacteria, those that don’t require oxygen for growth, can produce toxins, enzymes, gas, and acid, all of which destroy the surrounding tissue. Some examples are Staphylococci or Streptococci. Once the process has begun, the bacteria have momentum, with these enzymes facilitating the spread of disease.

  Bacteria can produce harmful substances called toxins, meant to either attack other bacteria in their vicinity or damage the host they have settled in. Typically, toxins cause degradation or lysis of cells, or trigger destructive immune responses.

  For some diseases, symptoms can be fully attributed to toxin production, with damage occurring right where the infection is. In other cases, such as with tetanus or certain staphylococci-associated infections, the toxin may travel through the bloodstream and cause symptoms somewhere else in the body.

The components that make up the bacterial cell wall, in particular, can set off a powerful chain reaction within the immune system. For instance, during an infection of gram-positive bacteria, the peptidoglycan and the products it breaks down into, can stimulate a fever or inflammation with devastating effects on the body. Or, lipopolysaccharide produced by gram-negative bacteria is categorised as endotoxin.

In low doses, endotoxin can activate the immune system or protective responses such as a fever. In higher doses, endotoxin can trigger extremely high fever, shock, or skin lesions, which can be deadly.

Exotoxin proteins, on the other hand, can be produced by either gram-positive or gram-negative bacteria. Proteins that fall into this category include those that cause cytolysis, which causes a cell to burst from osmotic pressure, or receptor-binding proteins that either cause cell death, or change their function altogether.Exotoxins are often encoded on a plasmid or a phage.

  Another category of toxins is super antigens, which activate the immune system to a life-threatening degree, causing toxic shock syndrome.

Finally, bacteria have developed multiple mechanisms to escape our host defenses, especially in the case of long-term infections. They might alter their surface proteins to evade detection, like Neisseria gonorrhoeae, physically hide within cells in the body,or inactivate our standard antibacterial defenses.

One of the most powerful virulence factors that some bacteria have are slime layers called capsules. These capsules can mimic the surface of a host cell, shielding the bacteria from typical immune responses. Other bacteria create makeshift shields within the site of infection, like Staphylococcus aureus, which forms a barrier using coagulase.

Ultimately, bacteria have a wide array of trick's up their sleeves. Some might express one virulence mechanism, while others might express several in tandem. Altogether, these mechanisms are ultimately what trigger disease symptoms. 


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