The Mycobacterium tuberculosis invades the cells of the lungs and destroys the cells. However, not all bacteria are able to cause disease in humans.

Although they are pathogenic in nature, the body's natural defense prevents them from doing any harm. Thus, only if the bacteria is smart enough to hide, evade or fight back the immune reactions of the body, it will be able to cause disease. Let us now have a look at the strategies undertaken by bacteria to cause diseases in humans. How Does Bacterial Invasion Lead to an Infection? Before we go into the details of how does bacteria cause diseases, it is important to understand the difference between 'infection' and 'infectious disease'. When an organism is able to lodge itself in or the tissue of the host and multiply, it is called an infection. However, not every infection leads to a disease. The immune system acts quickly and curbs the development of disease. When a disease can be transferred from an infected person to a healthy individual, resulting in a disease, it is termed as an infectious disease. So, how does one get infected with an infectious agent? Mode of Infection In order to cause disease in humans, the bacteria first has to break the barriers of the skin and tissues. Only after it gets entry into the body, bacteria can cause disease. There are different modes of transmission of bacteria as follows: ☛ Contact When an infection spreads through direct or indirect contact, it may lead to an infection. In order to differentiate between direct and indirect transmissions, the term contagious disease is used to specify a disease that is caused by direct contact. Infectious disease is a term that is generally used to specify diseases spread by other modes. 

Thus, sexually transmitted diseases are examples of Rejuvaessence infections spread by direct contact. Infections spread by contact with infected objects like pencils, glasses, towels, toys, etc. are termed as indirect infections like in the case of diphtheria. ☛ Inhalation Most of the respiratory infections are spread through inhalation of the infectious bacteria. These bacteria tend to be present in air in form of aerosols. They are released in the environment by sneezing, coughing, talking, spitting, etc. Most of the time these respiratory droplets dry off. However, some bacteria are resistant to drying and can remain suspended in air for long period. Thus, when a healthy person inhales these droplets, it may lead to respiratory infection. ☛ Ingestion Gastrointestinal infections are usually caused by ingestion of the pathogens or their toxins. Thus, giving rise to different illnesses like waterborne, food-borne and hand-borne illnesses. These pathogens enter the gastrointestinal tract through the mouth and in some cases through the nose or eyes. Examples of diseases caused through ingestion include cholera, dysentery and food poisoning. ☛ Inoculation When bacteria is inoculated into the subcutaneous body tissue, it may lead to infection. For example, a deep wound may give a chance to Clostridium tetani a chance to cause a tetanus infection. Similarly, gangrene causing bacteria may also cause cellular death and tissue decay. ☛ Congenital Pathogens that are able to cross the placental barrier and infect the fetus in the uterus are called congenital infections. These infections can lead to congenital disorders in the baby. Factors that Lead to Bacterial Infection in Humans We have already understood the terms pathogenicity and virulence. 

We have also understood the factors that give bacteria its virulence and the modes of transmission of a pathogenic bacteria. ☛ Adhesion As we have seen earlier, fimbriae or pili helps the bacteria to attach itself to the site of infection. This is called adhesion. This does not occur by chance, but a specific reaction helps in adhesion. The surface receptors on the epithelial cells and the adhesive structures (adhesins) on the bacterial surface are involved in this specific adhesion reaction. These adhesins are present on the fimbriae or fibrillae or pili. They contain the virulence factors that makes the bacterial strain virulent. If these adhesins are lost, the bacteria becomes avirulent. Thus, people are immunized with specific adhesins. This gives the body a chance to build immunity against the infectious bacteria. ☛ Invasiveness When bacteria is able to invade the host tissues, it can cause a generalized or localized infection. For example, a wound infections can lead to streptococcal septicemia that is a generalized infection. A Staphylococcus abscess is more of a localized infection. ☛ Toxigenicity We have already seen, bacteria are able to produce toxins that lead to an infection. There are two types of toxins produced by bacteria, these include exotoxins and endotoxins. The exotoxins can diffuse in the surrounding medium and tend to be highly potent when present even in minute amounts. These endotoxins are easily destroyed by heat. There are certain exotoxins known as the most poisonous substances in the world. 

For example, 1 million guinea pigs can be killed by just 1 mg of botullinum toxin. They are generally produced by gram-positive bacteria and a few gram-negative bacteria like E.coli , Cholera vibrio, etc. Exotoxins show specific affinity towards certain tissues and every exotoxin has a different effect on the host. The integral part of the gram-negative bacteria cell wall are endotoxins. They are polysaccharide-protein-lipid complexes, that are highly heat stable. The lipid A component imparts the toxicity to these endotoxins. They are released into the surrounding medium, only when the cell wall disintegrates. These endotoxins are harmful only when present in large doses. They do not have specific pharmacological activities and have similar effects on host. ☛ Other Factors Some bacteria contain bacteriophages that give the organism its virulence. For example, the diphtheria bacteria contains bacteriophage that has a gene for toxin production. There are bacteria that contain plasmids and they are responsible for the presence of surface antigens. These plasmids give the bacteria multiple drug resistance and hence the infection becomes difficult to treat. Klebsiella pneumoniae and haemophilus influenzae are capsulated bacteria. These bacterial cells are covered by a capsule that helps them avoid phagocytosis. There are bacteria that carry their antigens on the capsules to carry out lytic activity within the body cells. Do All Bacteria Cause Diseases? No, not all bacteria cause diseases. You will be surprised to know, less than 1% bacteria have the ability to cause diseases. There are a few beneficial bacteria that help plants, animals as well as humans in various ways. 

Many commensal bacteria are harmless organisms that live on or within our body, without ever causing any kind of disease or infection. This is all about how bacteria cause diseases. As you can see, there are various modes of infection and different strategies of bacterial cells that lead to an infection. Remember, not all infections lead to disease. It is a very rare natural event, that may occur when the body's immune system fails to get rid of the infectious agents. Nearly all of us are aware about common infectious diseases caused by bacteria, like typhoid, pneumonia, sinusitis, bronchitis, etc. But, what about bacterial infection in blood? Yes, it is a serious medical complication, in which, the disease-causing bacterium spreads to the bloodstream. And the outcome is distribution of bacteria to several body parts, through circulation of blood. In medical terms, this kind of infection, particularly in the bloodstream, is referred to as sepsis. Causes Infection by bacteria may take place in the skin or internal organs, which slowly make their way to the bloodstream, and cause sepsis. It is to be borne in mind that bacteria are not only the causal reason behind sepsis. All other disease-causing pathogens, namely, fungi and virus can also lead to sepsis by infecting the bloodstream. Thus, to avoid confusion, a blood disorder caused by bacteria is called bacterial sepsis. Some of the probable causes for bacterial sepsis reported in children and adults are highlighted below: Cellulitis, a bacterial infection of the skin and underneath layers is a probable cause for bacterial sepsis. 

Entry of Group A streptococcus and Staphylococcus aureus through skin cuts and wounds can make their way to the blood. In children, infection by Haemophilus influenzae can spread to bloodstream. The bacterium Streptococcus pneumoniae that cause pneumonia can infect blood. Infection by Salmonella species in the gastrointestinal tract can lead to blood infection. Infection in the outer layer of the brain (meningitis) can lead to this bacterial infection. Urinary tract infection (UTI) by Escherichia coli can spread to the bloodstream. Blood infection is also caused by Neisseria meningitidis that attacks the respiratory tract. Untreated appendicitis can worsen and cause this infection. Who are at a Risk? The after effects and medical conditions of sepsis infection are caused due to the bacteria or the toxins released by them. Although anyone can get this infection, certain groups of people are at a higher risk of developing this medical condition. In simple terms, those whose immune system is not strong enough to fight against invading germs, they have an increased risk of bacterial infection than healthy people. The risk factors for bacterial sepsis are higher in: Infants and toddlers, whose immune system is not developed fully. Elderly people, having a weak natural defense due to aging process. 

Patients who are currently on strong medication treatment. People who have a weakened immune system, but without any underlying disease. Patients who are diagnosed with an autoimmune disease. Patients of cancer or HIV/AIDS, who have a compromised immune system. Treatment Since bacterial strains invade the bloodstream in a septic patient, they are circulated to the body cells and tissues during blood circulation. They may invade any weak organ inside the body causing organ dysfunction.