Bacteria cause diseases in humans when they

Índice Show

When do bacteria cause disease?
Managing bacterial infections
Identifying the disease
Selected Resources
How bacteria and viruses enter the body
Viruses are spread from one person to another by:
Bacteria types
Characteristics of the bacterium
Curing a bacterial infection
Virus types
The body’s response to viral infection
Curing a viral infection
Immunisation against viral infection is not always possible
Where to get help
Things to remember

Throughout history, bacterial infections have played a central role in the lives and deaths of humans.

Bacteria cause many common infections such as pneumonia, wound infections, bloodstream infections (sepsis) and sexually transmitted diseases like gonorrhea. Moreover, they have been responsible for several major disease epidemics. One example from the 1340’s is the plague, also known as the “Black death”, that spread across Asia and Europe along the trading routes, killing millions of people. The disease is now known to be caused by the bacterium Yersinia pestis and is treatable by antibiotics.

When do bacteria cause disease?

Bacteria can be strictly pathogenic, which means that they will cause disease if they manage to overwhelm the human immune system. Other bacteria only cause disease given the right circumstances, these are so-called opportunistic pathogens. Opportunistic pathogens normally do not cause infections in healthy humans, but the risk increases when the immune system is compromised or suppressed by for example cancer chemotherapy, malnutrition or other diseases (like HIV/AIDS). These infections often originate from the individual’s own bacterial flora such as that on the skin or in the gut.

Many bacterial pathogens can spread via water and food, including Salmonella, Campylobacter and E. coli. Sometimes bacteria are transmitted directly or indirectly from animals to humans and cause disease. Such infections are called zoonotic infections. Other bacteria like Neisseria gonorrhea and Chlamydia trachomatis spread via sexual contacts.

Managing bacterial infections

The introduction of antibiotics to treat bacterial infections in combination with improved hygiene and sanitation, use of vaccinations and increased knowledge about bacteria have greatly reduced deaths from bacterial diseases. However, antibiotic resistance is now threatening to again leave us without effective treatments for many common bacterial infections. Resistant bacteria are widespread in most parts of the world and more and more people die from bacterial infections because the antibiotics have stopped working. For information about antibiotic resistance, see Antibiotic resistance, and the sub-sections that follows.

Identifying the disease

Diagnostics are a set of tools used to determine the nature and causes of disease. This information is then used to determine what the appropriate therapy is. It could be, for example, a specific antibiotic, other medicines or bed rest. In the absence of a proper diagnosis, many patients may not get the correct antibiotic treatment when they need it. Another common problem is that patients who do not need antibiotics are given one just in case it is a bacterial infection.

Below are a set of resources about bacterial diseases and their spread.

Selected Resources

Many human illnesses are caused by infection with either bacteria or viruses.
Most bacterial diseases can be treated with antibiotics, although antibiotic-resistant strains are starting to emerge.
Viruses pose a challenge to the body’s immune system because they hide inside cells.
It is possible to be vaccinated against some of the major disease-causing viruses (such as measles and polio), as well as bacterial diseases such as Hemophilus influenza Type b (Hib), tetanus and whooping cough.

How bacteria and viruses enter the body

To cause disease, pathogenic bacteria must gain access into the body. The range of access routes for bacteria includes:

Cuts
Contaminated food or water
Close contact with an infected person
Contact with the faeces of an infected person
Breathing in the exhaled droplets when an infected person coughs or sneezes
Indirectly, by touching contaminated surfaces – such as taps, toilet handles, toys and nappies.

Viruses are spread from one person to another by:

Coughs
Sneezes
Vomits
Bites from infected animals or insects
Exposure to infected bodily fluids through activities such as sexual intercourse or sharing hypodermic needles.

Forgetting to wash your hands after handling pets and animals is another way for germs to be taken in by mouth.

Bacteria types

Bacteria that cause disease are broadly classified according to their shape. The four main groups include:

Bacilli – shaped like a rod with a length of around 0.03mm. Illnesses such as typhoid and cystitis are caused by bacilli strains.
Cocci – shaped like a sphere with a diameter of around 0.001mm. Depending on the sort, cocci bacteria group themselves in a range of ways, such as in pairs, long lines or tight clusters. Examples include Staphylococci (which cause a host of infections including boils) and Gonococci (which cause the sexually transmissible infection gonorrhoea).
Spirochaetes – as the name suggests, these bacteria are shaped like tiny spirals. Spirochaetes bacteria are responsible for a range of diseases, including the sexually transmissible infection syphilis.
Vibrio – shaped like a comma. The tropical disease cholera, characterised by severe diarrhoea and dehydration, is caused by the vibrio bacteria.

Characteristics of the bacterium

Most bacteria, apart from the cocci variety, move around with the aid of small lashing tails (flagella) or by whipping their bodies from side to side. Under the right conditions, a bacterium reproduces by dividing in two. Each ‘daughter’ cell then divides in two and so on, so that a single bacterium can bloom into a population of some 500,000 or more within just eight hours.

If the environmental conditions don’t suit the bacteria, some varieties morph into a dormant state. They develop a tough outer coating and await the appropriate change of conditions. These hibernating bacteria are called spores. Spores are harder to kill than active bacteria because of their outer coating.

Curing a bacterial infection

The body reacts to disease-causing bacteria by increasing local blood flow (inflammation) and sending in cells from the immune system to attack and destroy the bacteria. Antibodies produced by the immune system attach to the bacteria and help in their destruction. They may also inactivate toxins produced by particular pathogens, for example tetanus and diphtheria.

Serious infections can be treated with antibiotics, which work by disrupting the bacterium’s metabolic processes, although antibiotic-resistant strains are starting to emerge. Immunisation is available to prevent many important bacterial diseases such as Hemophilus influenza Type b (Hib), tetanus and whooping cough..

Virus types

A virus is a miniscule pocket of protein that contains genetic material. If you placed a virus next to a bacterium, the virus would be dwarfed. For example, the polio virus is around 50 times smaller than a Streptococci bacterium, which itself is only 0.003mm long. Viruses can be described as either RNA or DNA viruses, according to which type of nucleic acid forms their core.

The four main types of virus include:

Icosahedral – the outer shell (capsid) is made from 20 flat sides, which gives a spherical shape. Most viruses are icosahedral.
Helical – the capsid is shaped like a rod.
Enveloped – the capsid is encased in a baggy membrane, which can change shape but often appears spherical.
Complex – the genetic material is coated, but without a capsid.

The body’s response to viral infection

Viruses pose a considerable challenge to the body’s immune system because they hide inside cells. This makes it difficult for antibodies to reach them. Some special immune system cells, called T-lymphocytes, can recognise and kill cells containing viruses, since the surface of infected cells is changed when the virus begins to multiply. Many viruses, when released from infected cells, will be effectively knocked out by antibodies that have been produced in response to infection or previous immunisation.

Curing a viral infection

Antibiotics are useless against viral infections. This is because viruses are so simple that they use their host cells to perform their activities for them. So antiviral drugs work differently to antibiotics, by interfering with the viral enzymes instead.

Antiviral drugs are currently only effective against a few viral diseases, such as influenza, herpes, hepatitis B and C and HIV – but research is ongoing. A naturally occurring protein, called interferon (which the body produces to help fight viral infections), can now be produced in the laboratory and is used to treat hepatitis C infections.

Immunisation against viral infection is not always possible

It is possible to vaccinate against many serious viral infections such as measles, mumps, hepatitis A and hepatitis B. An aggressive worldwide vaccination campaign, headed by the World Health Organization (WHO), managed to wipe out smallpox. However, some viruses – such as those that cause the common cold – are capable of mutating from one person to the next. This is how an infection with essentially the same virus can keep dodging the immune system. Vaccination for these kinds of viruses is difficult, because the viruses have already changed their format by the time vaccines are developed.

Where to get help

Your doctor
Your pharmacist

Things to remember

Many human illnesses are caused by infection with either bacteria or viruses.
Most bacterial diseases can be treated with antibiotics, although antibiotic-resistant strains are starting to emerge.
Viruses pose a challenge to the body’s immune system because they hide inside cells.
It is possible to be vaccinated against some of the major disease-causing viruses (such as measles and polio), as well as bacterial diseases such as Hemophilus influenza Type b (Hib), tetanus and whooping cough.

Infectious Diseases, Department of Health & Human Services, Victorian Government.

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