Pathogens and Infectious Disease
3. Bacterial Diseases
Learning outcomes
- I can identify examples of bacterial diseases.
- I can describe how bacterial infections develop.
- I can explain common symptoms of bacterial diseases.
- I can investigate methods used to treat bacterial infections.
- I can explain the importance of preventing bacterial disease.
What Is a Bacterial Disease?
A bacterial disease is an illness caused by pathogenic bacteria infecting a host.
Bacteria are microscopic, single-celled:
prokaryotic organisms.
Most bacteria do not cause disease. Many are harmless or beneficial.
However, some bacteria are:
pathogenic.
These bacteria can enter the body, reproduce, damage tissues, produce toxins, or trigger harmful inflammatory responses.
Bacteria Are Living Cells
Unlike viruses, bacteria are:
cells.
A typical bacterial cell contains:
- cell membrane
- cytoplasm
- ribosomes
- circular DNA
- cell wall
Some bacteria also have:
- flagella
- capsules
- plasmids
Bacteria do not contain a membrane-bound:
nucleus.
Not All Bacteria Cause Disease
It is important not to think of all bacteria as:
harmful.
Many bacteria are essential to life on Earth.
Bacteria are involved in:
- decomposition
- nutrient cycling
- digestion
- food production
- biotechnology
- maintaining healthy microbial communities
Only certain bacterial species or strains are capable of causing particular:
diseases.
Examples of Bacterial Diseases
Important examples include:
- tuberculosis
- cholera
- salmonellosis
- bacterial meningitis
- strep throat
- gonorrhea
- syphilis
- tetanus
- whooping cough
- some forms of pneumonia
Different bacterial diseases affect different parts of the body and spread in different ways.
Tuberculosis
Tuberculosis, usually called TB, is caused by the bacterium:
Mycobacterium tuberculosis.
TB most commonly affects the:
lungs,
although it can affect other parts of the body.
How Tuberculosis Spreads
TB spreads through:
airborne respiratory particles.
A person with infectious TB disease in the lungs or throat can release bacteria into the air.
Another person may become infected after inhaling these bacteria.
Transmission is more likely with:
- prolonged exposure
- close indoor contact
- poor ventilation
Symptoms of Tuberculosis
Active pulmonary TB can cause symptoms such as:
- persistent cough
- chest pain
- coughing up blood or sputum
- fever
- night sweats
- fatigue
- loss of appetite
- weight loss
Not everyone infected with TB bacteria develops active disease.
Latent TB Infection
Some people become infected with TB bacteria but do not become:
ill.
The immune system keeps the bacteria under control.
This is called:
latent TB infection.
A person with latent TB infection does not have active TB symptoms and does not spread TB bacteria in the same way as someone with infectious pulmonary TB disease.
However, latent infection can sometimes later develop into:
active TB disease.
Cholera
Cholera is a bacterial disease caused by:
Vibrio cholerae.
It mainly affects the:
small intestine.
The bacteria produce a toxin that causes large amounts of water and electrolytes to move into the intestine.
How Cholera Spreads
Cholera is commonly transmitted through:
contaminated food or water.
It is strongly associated with inadequate:
- sanitation
- sewage treatment
- access to clean drinking water
The disease spreads mainly through the:
fecal-oral route.
Symptoms of Cholera
Cholera can cause:
- severe watery diarrhea
- vomiting
- dehydration
- electrolyte loss
- weakness
Severe dehydration can become life-threatening if not treated quickly.
Treating Cholera
The most important treatment for cholera is replacing lost:
water and electrolytes.
This is commonly achieved using:
oral rehydration solution (ORS).
Severe cases may require intravenous fluids.
Antibiotics may be used in some cases, particularly for severe disease, but replacing fluids is the central treatment.
Salmonellosis
Salmonellosis is an infection caused by certain:
Salmonella bacteria.
It commonly affects the:
digestive system.
Transmission is often associated with contaminated food.
How Salmonella Spreads
Salmonella may be transmitted through:
- contaminated food
- contaminated water
- contact with infected animals
- cross-contamination during food preparation
Foods associated with transmission can include improperly handled or undercooked animal products and contaminated produce.
Symptoms of Salmonellosis
Symptoms can include:
- diarrhea
- fever
- stomach cramps
- nausea
- vomiting
Many otherwise healthy people recover without specific antibiotic treatment.
Severe infections may require medical care.
Preventing Salmonella Infection
Risk can be reduced by:
- washing hands
- cooking foods appropriately
- avoiding cross-contamination
- storing food safely
- cleaning food-preparation surfaces
- using safe drinking water
Food hygiene is therefore an important method of preventing:
bacterial disease.
Bacterial Meningitis
Meningitis is inflammation of the membranes surrounding the brain and spinal cord.
These membranes are called the:
meninges.
Several different bacteria can cause bacterial meningitis.
Symptoms of Bacterial Meningitis
Symptoms can include:
- severe headache
- fever
- stiff neck
- nausea
- sensitivity to light
- confusion
Some cases may also involve a rash.
Bacterial meningitis can become serious rapidly and requires:
urgent medical treatment.
Strep Throat
Strep throat is commonly caused by:
Streptococcus pyogenes.
It affects the:
throat and tonsils.
Symptoms may include:
- sore throat
- pain when swallowing
- fever
- swollen lymph nodes
- red or swollen tonsils
Not every sore throat is caused by bacteria.
Many sore throats are caused by:
viruses.
Tetanus
Tetanus is caused by:
Clostridium tetani.
The bacterium can produce a powerful:
neurotoxin.
The spores of the bacterium are commonly found in the environment, including soil.
They can enter the body through:
wounds.
How Tetanus Causes Disease
Clostridium tetani can grow under suitable low-oxygen conditions in contaminated tissue.
It produces a toxin that interferes with the:
nervous system.
This can cause:
- muscle stiffness
- painful muscle spasms
- difficulty swallowing
- jaw stiffness
Tetanus does not normally spread directly from person to person.
Preventing Tetanus
Tetanus can be prevented very effectively through:
vaccination.
Proper wound care is also important.
This demonstrates that preventing bacterial disease does not always involve stopping person-to-person transmission.
Sometimes prevention focuses on preventing a pathogen from entering or establishing itself in the:
body.
Whooping Cough
Whooping cough, or pertussis, is caused by:
Bordetella pertussis.
It is a respiratory disease.
The bacteria spread between people through:
respiratory transmission.
Symptoms can include severe repeated coughing episodes.
Vaccination is an important method of prevention.
Gonorrhea
Gonorrhea is caused by:
Neisseria gonorrhoeae.
It is primarily transmitted through:
sexual contact.
Some infected people have noticeable symptoms, while others may have few or no symptoms.
Untreated infection can sometimes cause serious complications.
Syphilis
Syphilis is caused by the bacterium:
Treponema pallidum.
It is commonly transmitted through:
sexual contact.
It can also be transmitted during pregnancy to a developing fetus.
The disease develops through different stages and can cause serious complications if untreated.
How Does a Bacterial Infection Begin?
A bacterial infection generally requires several steps.
A simplified sequence is:
exposure → entry → attachment or colonization → multiplication → damage → symptoms
Not every exposure successfully completes all these stages.
Step 1: Exposure
First, a person must encounter the:
pathogenic bacterium.
Exposure might occur through:
- air
- food
- water
- direct contact
- sexual contact
- wounds
- contaminated objects
- vectors in some bacterial diseases
Exposure does not automatically mean infection.
Step 2: Entry
The bacteria must enter through a suitable:
portal of entry.
Possible entry points include:
- respiratory tract
- digestive tract
- reproductive tract
- broken skin
- bloodstream
Different bacterial species are adapted to different entry routes.
Step 3: Attachment
Many bacteria must attach to:
host cells or tissues.
Some bacteria possess surface structures that help them bind to particular cells.
Successful attachment can prevent the bacteria from simply being:
removed.
Step 4: Colonization
The bacteria begin establishing themselves within the:
host.
They may obtain nutrients and reproduce.
This process is called:
colonization.
Colonization does not always cause disease, because some bacteria can live on or in the body harmlessly.
Step 5: Multiplication
Under suitable conditions, bacteria reproduce by:
binary fission.
One bacterial cell divides into:
two cells.
Those cells can divide again.
A simplified sequence is:
1 → 2 → 4 → 8 → 16 → 32...
Under favorable conditions, bacterial populations can increase rapidly.
Step 6: Tissue Damage
Some bacteria directly invade and damage:
host tissues.
As bacteria multiply, they may interfere with the normal functions of cells and organs.
Damage can contribute to symptoms such as:
- pain
- swelling
- inflammation
- impaired organ function
Step 7: Toxin Production
Some pathogenic bacteria produce:
toxins.
Toxins are substances that damage cells or interfere with normal physiological processes.
For example:
Vibrio cholerae produces cholera toxin.
Clostridium tetani produces tetanus neurotoxin.
In these diseases, toxins play a major role in producing symptoms.
Exotoxins
Some bacteria release proteins called:
exotoxins.
These toxins can act on particular cells or tissues.
Their effects may be very powerful even when relatively small amounts are present.
Examples include toxins associated with:
- tetanus
- diphtheria
- cholera
- botulism
The Immune Response
Some symptoms of bacterial disease are caused partly by the body's:
immune response.
When immune cells recognize infection, they release chemical signals.
These can cause:
- inflammation
- fever
- swelling
- pain
- fatigue
These responses can help fight infection, but they can also make a person feel ill.
Why Does Fever Occur?
Fever is an increase in body temperature associated with changes in temperature regulation during illness.
Certain immune signals can cause the body's temperature set point to:
increase.
Fever is therefore often part of the body's response to infection rather than something directly produced by bacterial growth alone.
Common Symptoms of Bacterial Disease
Symptoms vary greatly depending on the infection.
Possible symptoms include:
- fever
- fatigue
- pain
- inflammation
- swelling
- cough
- diarrhea
- vomiting
- sore throat
- skin redness
- discharge
There is no single symptom that proves an illness is:
bacterial.
Symptoms Depend on the Site of Infection
A respiratory bacterial infection may cause:
cough and breathing problems.
An intestinal infection may cause:
diarrhea and abdominal pain.
A skin infection may cause:
redness, swelling, warmth, and pain.
A urinary infection may cause:
pain or burning during urination.
The location of infection strongly influences the symptoms.
Bacterial vs Viral Disease
Bacterial and viral diseases can produce similar:
symptoms.
For example, both may cause:
- fever
- cough
- fatigue
- inflammation
Therefore, symptoms alone may not always identify whether an infection is bacterial or viral.
Medical testing may sometimes be necessary.
Diagnosing Bacterial Infections
Doctors may use several methods to identify bacterial infections.
These can include:
- examination of symptoms
- patient history
- microscopy
- bacterial culture
- biochemical tests
- antigen tests
- molecular tests such as PCR
The appropriate method depends on the disease.
Bacterial Culture
A bacterial culture involves growing bacteria from a sample under controlled laboratory conditions.
Samples might come from:
- blood
- urine
- sputum
- throat swabs
- wounds
Growing the bacteria can help identify the organism responsible for an infection.
Antibiotics
Antibiotics are medicines used to treat susceptible:
bacterial infections.
Different antibiotics interfere with different bacterial structures or processes.
Possible targets include:
- cell-wall synthesis
- protein synthesis
- DNA replication
- metabolic pathways
Antibiotics Do Not Treat Viruses
Antibiotics work against:
bacteria.
They do not treat diseases caused by:
viruses.
Viruses do not possess many of the structures and metabolic processes targeted by antibiotics.
For example, antibiotics do not treat ordinary viral colds or influenza.
Different Antibiotics Work Differently
An antibiotic effective against one bacterial species may not work against:
another.
Doctors may therefore consider:
- the likely bacterial species
- the location of infection
- the severity of disease
- local resistance patterns
- laboratory results
when selecting treatment.
Antibiotic Susceptibility Testing
A laboratory can sometimes test bacteria against different:
antibiotics.
The aim is to determine which medicines are likely to inhibit or kill the bacteria.
This can help doctors select an effective:
treatment.
Antibiotic Resistance
Antibiotic resistance occurs when bacteria evolve characteristics that allow them to survive exposure to an antibiotic that would normally inhibit or kill them.
The antibiotic does not cause individual bacteria to deliberately become resistant.
Instead:
genetic variation exists → antibiotic creates selection pressure → susceptible bacteria are removed → resistant bacteria survive and reproduce
This is an example of:
natural selection.
How Resistance Spreads
Resistance can arise through:
mutation.
Resistance genes can also sometimes move between bacteria through:
horizontal gene transfer.
As resistant bacteria reproduce and spread, resistant infections can become more common.
Why Antibiotic Resistance Matters
Antibiotic resistance can make infections:
- harder to treat
- longer lasting
- more expensive to manage
- more likely to cause complications
It can also reduce the number of effective treatment options.
This makes responsible antibiotic use extremely important.
Responsible Antibiotic Use
Antibiotics should be used appropriately.
Important principles include:
- use antibiotics when medically indicated
- use the correct antibiotic when possible
- follow professional instructions
- do not use antibiotics to treat viral illnesses
- avoid unnecessary antibiotic use
Responsible use helps reduce unnecessary selection pressure for resistance.
Treatment Is Not Always Antibiotics
Not every bacterial infection requires antibiotic treatment.
Some mild infections may resolve with:
supportive care and the immune response.
Other bacterial diseases require urgent antibiotics.
Some diseases also require other treatments.
For example, cholera treatment focuses heavily on:
rehydration.
Supportive Treatment
Supportive treatment helps the body while it fights the infection.
Depending on the illness, this might include:
- fluids
- electrolyte replacement
- rest
- nutritional support
- fever or pain management
Supportive treatment does not necessarily kill the bacteria directly.
Preventing Bacterial Disease
Preventing infection is often easier and safer than treating serious disease.
Prevention methods include:
- handwashing
- clean water
- sanitation
- safe food preparation
- vaccination
- safe sexual practices
- wound care
- respiratory hygiene
- appropriate infection-control practices
The best strategy depends on the bacterium's:
mode of transmission.
Handwashing
Proper handwashing can reduce transmission of many pathogens.
Soap and water help physically remove microorganisms from the:
hands.
Handwashing is particularly important:
- after using the toilet
- before preparing food
- before eating
- after handling potentially contaminated materials
Clean Water and Sanitation
Clean water and effective sanitation are especially important for preventing:
fecal-oral bacterial diseases.
For example:
contaminated water → pathogen enters digestive system → infection
Water treatment and sewage management interrupt this transmission pathway.
Food Safety
Foodborne bacterial disease can be reduced through:
- proper cooking
- safe refrigeration
- handwashing
- clean preparation surfaces
- separation of raw and cooked foods
- prevention of cross-contamination
These practices reduce bacterial survival and transfer.
Vaccination
Vaccines are available against some bacterial diseases.
Examples include vaccines that protect against:
- tetanus
- diphtheria
- pertussis
- some forms of bacterial meningitis
- some forms of pneumonia
- tuberculosis in certain settings
Vaccination prepares the immune system to respond more effectively to particular pathogens or their components.
Preventing Respiratory Bacterial Disease
For bacterial diseases spread through respiratory routes, prevention can include:
- appropriate ventilation
- respiratory hygiene
- reducing exposure to infectious individuals when appropriate
- vaccination where available
- early diagnosis and treatment in some diseases
The most effective combination depends on the particular pathogen.
Preventing Sexually Transmitted Bacterial Disease
Strategies can include:
- barrier protection
- testing
- appropriate treatment
- informing relevant partners according to medical guidance
- reducing exposure to infection
Some bacterial sexually transmitted infections can be treated with antibiotics, although antibiotic resistance is an increasing concern for some pathogens.
Why Prevention Protects Communities
Preventing infection does more than protect one:
individual.
If fewer people become infected, there are fewer opportunities for the pathogen to:
spread.
Therefore:
fewer infections → fewer infectious sources → fewer transmission opportunities
Prevention can protect both individuals and populations.
Breaking the Chain of Infection
Bacterial disease prevention can target different parts of the:
chain of infection.
For example:
Pathogen
Antibiotics may eliminate susceptible bacteria in infected individuals.
Transmission route
Handwashing, sanitation, and food safety can interrupt spread.
Susceptible host
Vaccination can reduce susceptibility to particular diseases.
Disease control is often most effective when several approaches are combined.
Worked Example 1
A patient develops severe watery diarrhea after drinking contaminated water.
A possible bacterial disease is:
cholera.
The pathogen is:
Vibrio cholerae.
The likely transmission route is:
fecal-oral transmission through contaminated water.
Worked Example 2
A bacterium enters a wound and produces a toxin affecting the nervous system.
A possible disease is:
tetanus.
The bacterium is:
Clostridium tetani.
An important prevention method is:
vaccination.
Worked Example 3
A patient has a persistent cough, night sweats, fever, and weight loss.
One possible disease that would need medical investigation is:
tuberculosis.
Symptoms alone, however, are not enough to make a diagnosis.
Worked Example 4
A person has a sore throat.
Should antibiotics automatically be used?
No.
Sore throats can have bacterial or viral causes.
The cause should be assessed appropriately before antibiotics are used when treatment decisions depend on it.
Worked Example 5
A bacterial population contains a few bacteria with an antibiotic-resistance mutation.
An antibiotic is introduced.
Susceptible bacteria are killed or inhibited.
Resistant bacteria survive.
They reproduce.
Over time:
the proportion of resistant bacteria increases.
This demonstrates natural selection.
Worked Example 6
A disease spreads mainly through contaminated drinking water.
Which prevention strategy would probably have the greatest direct effect on transmission?
Improving:
clean water and sanitation.
The strategy targets the disease's transmission route.
Comparing Four Bacterial Diseases
Tuberculosis
Cause:
Mycobacterium tuberculosis
Main transmission:
respiratory
Commonly affects:
lungs
Important prevention approaches:
early detection, appropriate treatment, ventilation, and vaccination in settings where recommended
Cholera
Cause:
Vibrio cholerae
Main transmission:
contaminated food or water
Commonly affects:
intestines
Important prevention:
clean water and sanitation
Salmonellosis
Cause:
Salmonella bacteria
Common transmission:
contaminated food
Commonly affects:
digestive system
Important prevention:
food hygiene
Tetanus
Cause:
Clostridium tetani
Entry:
contaminated wounds
Major effect:
neurotoxin affects the nervous system
Important prevention:
vaccination and wound care
Common Mistake: All Bacteria Cause Disease
Most bacteria do:
not.
Many bacteria are harmless or beneficial.
Only particular bacteria cause specific bacterial diseases.
Common Mistake: All Bacterial Diseases Spread Between People
Some do, but others do not normally spread directly from person to person.
For example:
tetanus usually results from environmental bacterial spores entering wounds.
Common Mistake: Antibiotics Kill Viruses
Antibiotics act against:
bacteria.
They do not kill viruses responsible for illnesses such as influenza or the common cold.
Common Mistake: Every Bacterial Infection Needs Antibiotics
Not necessarily.
Treatment depends on:
- the bacterium
- severity
- infection site
- patient's circumstances
- likelihood of complications
Medical professionals determine when antibiotic treatment is appropriate.
Common Mistake: People Become Resistant to Antibiotics
It is the:
bacteria
that become resistant.
A person's body does not become antibiotic-resistant.
Common Mistake: Antibiotics Teach Bacteria to Become Resistant
Antibiotics do not intentionally teach bacteria anything.
They create:
selection pressure.
Bacteria with resistance characteristics are more likely to survive and reproduce.
Common Mistake: Symptoms Can Identify the Pathogen
Many different diseases produce similar:
symptoms.
Laboratory testing may be needed to identify the pathogen.
Check Your Understanding
1. Define a bacterial disease.
2. What type of cell is a bacterium?
3. Are all bacteria harmful? Explain.
4. Give four examples of bacterial diseases.
5. What bacterium causes tuberculosis?
6. How does tuberculosis commonly spread?
7. Give three symptoms of active pulmonary TB.
8. What is latent TB infection?
9. What bacterium causes cholera?
10. How is cholera commonly transmitted?
11. Why can cholera become dangerous?
12. What is the most important general treatment for severe fluid loss in cholera?
13. What bacteria cause salmonellosis?
14. How is Salmonella commonly transmitted?
15. Give three ways to reduce foodborne bacterial infection.
16. What is bacterial meningitis?
17. Why does suspected bacterial meningitis require urgent treatment?
18. What bacterium commonly causes strep throat?
19. Why should every sore throat not automatically be treated with antibiotics?
20. What bacterium causes tetanus?
21. How does tetanus usually enter the body?
22. How does tetanus cause disease?
23. What is an important method of preventing tetanus?
24. Describe the major stages through which a bacterial infection may develop.
25. What is colonization?
26. How do bacteria reproduce?
27. What is binary fission?
28. How can bacteria damage host tissues?
29. What is a bacterial toxin?
30. Explain how the immune response can produce symptoms.
31. What is an antibiotic?
32. Why do antibiotics not treat viral infections?
33. Why might doctors perform a bacterial culture?
34. What is antibiotic resistance?
35. Explain how natural selection can increase antibiotic resistance in a bacterial population.
36. Why is unnecessary antibiotic use a problem?
37. Give four methods of preventing bacterial diseases.
38. Explain why clean water and sanitation reduce some bacterial diseases.
39. Explain how vaccination can prevent certain bacterial diseases.
40. Explain why preventing bacterial disease is important for both individuals and communities.
Key Terms
- Bacterium: Single-celled prokaryotic organism.
- Bacterial disease: Disease caused by pathogenic bacteria.
- Pathogenic: Capable of causing disease.
- Infection: Establishment and multiplication of a pathogen within a host.
- Colonization: Establishment of microorganisms in or on a host.
- Binary fission: Process by which one bacterial cell divides into two.
- Toxin: Harmful substance capable of disrupting normal biological processes.
- Exotoxin: Toxin released by certain bacteria.
- Inflammation: Immune response involving changes such as redness, heat, swelling, and pain.
- Tuberculosis: Bacterial disease caused by Mycobacterium tuberculosis.
- Cholera: Intestinal bacterial disease caused by Vibrio cholerae.
- Salmonellosis: Infection caused by certain Salmonella bacteria.
- Tetanus: Disease caused by toxin produced by Clostridium tetani.
- Antibiotic: Medicine used to treat susceptible bacterial infections.
- Antibiotic resistance: Ability of bacteria to survive or grow despite an antibiotic that would normally inhibit or kill them.
- Natural selection: Process in which organisms with advantageous inherited characteristics are more likely to survive and reproduce.
- Bacterial culture: Laboratory growth of bacteria for investigation.
- Antibiotic susceptibility testing: Testing used to determine which antibiotics are effective against a bacterial isolate.
- Vaccination: Method of preparing the immune system to respond to a particular pathogen or antigen.
- Sanitation: Systems and practices for maintaining hygienic conditions and safely managing waste.
- Supportive treatment: Treatment that supports body functions without necessarily eliminating the pathogen directly.
Key Takeaways
- Bacterial diseases are caused by pathogenic bacteria.
- Most bacteria are not harmful.
- Bacteria are living, single-celled prokaryotes.
- Important bacterial diseases include tuberculosis, cholera, salmonellosis, tetanus, bacterial meningitis, pertussis, gonorrhea, and syphilis.
- Different bacterial diseases have different transmission routes.
- Bacterial infection can involve entry, attachment, colonization, multiplication, and tissue damage.
- Bacteria reproduce through binary fission.
- Some bacteria damage tissues directly.
- Others cause disease by producing toxins.
- The body's immune response can also produce symptoms such as fever and inflammation.
- Symptoms depend strongly on the location and type of infection.
- Symptoms alone cannot always distinguish bacterial infections from viral infections.
- Antibiotics are used to treat susceptible bacterial infections.
- Antibiotics do not treat viral infections.
- Not every bacterial infection requires antibiotics.
- Laboratory cultures and other tests can help identify bacterial infections.
- Antibiotic resistance occurs when bacteria survive antibiotics that would normally inhibit or kill them.
- Natural selection can increase the frequency of antibiotic resistance in bacterial populations.
- Responsible antibiotic use is important for slowing the development and spread of resistance.
- Prevention methods include hygiene, sanitation, clean water, food safety, vaccination, wound care, and appropriate infection-control measures.
- Prevention strategies should match the bacterium's mode of transmission.
- Preventing bacterial infections protects both individuals and the wider community.