Table of Contents
Overview: When Helpful Bacteria Become Harmful
Most bacteria are harmless or even essential for life (for example, gut flora or soil bacteria). Some species, however, can invade the human body, multiply, and damage tissues or disturb normal functions. These are called pathogenic (disease‑causing) bacteria.
This chapter focuses on:
- What makes certain bacteria pathogenic
- How they cause damage in the body
- Typical patterns of bacterial infections
- How bacterial diseases are diagnosed, treated, and prevented in general
Specific examples (plague, Salmonella) are treated in their own sections, so they are mentioned here only briefly.
Routes of Entry and Spread in the Body
Pathogenic bacteria must first reach the body and then overcome its defenses. Common portals of entry are:
- Respiratory tract
- Inhalation of droplets or aerosols (e.g., from coughing, sneezing, talking)
- Causes infections such as pneumonia, whooping cough, or tuberculosis
- Gastrointestinal tract
- Ingestion with contaminated water, food, or hands
- Must survive gastric acid and digestive enzymes
- Can cause diarrhea, vomiting, or systemic disease (e.g., cholera, typhoid)
- Urogenital tract
- Sexual contact, poor hygiene, or use of catheters
- Leads to urinary tract infections and sexually transmitted bacterial diseases
- Skin and mucous membranes
- Through wounds, burns, insect bites, injections, or medical devices
- Even small injuries can serve as entry points (e.g., for staphylococci)
- Transplacental and perinatal transmission
- Some bacteria can be passed from mother to child during pregnancy or birth
Once inside, bacteria can:
- Stay localized near the entry point (e.g., skin abscess)
- Spread via lymphatic vessels to lymph nodes
- Enter the bloodstream (bacteremia), possibly leading to sepsis
- Colonize specific organs (e.g., meninges, heart valves, joints)
Virulence Factors: Tools of Pathogenic Bacteria
Not all strains of a bacterial species are equally dangerous. The ability to cause disease depends on virulence factors—molecules or structures that help bacteria colonize the host, evade defenses, and damage tissues.
Important categories:
Adhesion and Colonization
To cause disease, bacteria must first attach to host surfaces and resist being washed away.
- Pili (fimbriae)
- Hairlike surface structures that bind to specific receptors on host cells
- Important in urinary tract infections and intestinal infections
- Adhesins
- Specialized proteins in the cell wall or membrane
- Determine which tissues a bacterium can colonize (tissue tropism)
- Biofilm formation
- Communities of bacteria embedded in a self‑produced slime (matrix)
- Occur on teeth (dental plaque), catheters, artificial joints
- Bacteria in biofilms are harder to eliminate and more resistant to antibiotics and immune attack
Invasion and Spread in Tissues
Some pathogens remain on surfaces; others invade deeper layers or even live inside host cells.
- Invasins
- Bacterial proteins that trigger uptake by host cells or breakdown of cell junctions and extracellular matrix
- Allow crossing of mucosal barriers and spread within tissues
- Enzymes that destroy tissue
- Hyaluronidase, collagenase, proteases, DNases
- Break down connective tissue or DNA in pus, “liquefying” tissue and facilitating spread
Evasion of the Immune System
To persist in the host, bacteria use strategies to avoid or resist immune defenses:
- Capsules
- Sugary (polysaccharide) coats around the cell
- Hinder phagocytosis (engulfment) by immune cells
- Important virulence factor of many severe pathogens (e.g., pneumococci)
- Antigenic variation
- Changing surface structures to avoid recognition by antibodies
- Leads to repeated or chronic infections
- Destruction or inactivation of immune components
- Enzymes that degrade antibodies
- Interference with complement activation (a protein cascade that helps kill bacteria)
- Intracellular lifestyle
- Some bacteria survive inside macrophages or other cells
- They are less accessible to antibodies and some antibiotics
Toxins
Bacterial toxins are central to many diseases. They can act locally or via the blood on distant organs.
Exotoxins
- Proteins actively secreted by living bacteria
- Often heat‑sensitive and highly specific in their action
- Even small amounts can be lethal
Common types:
- Cytotoxins
- Damage or kill host cells by disrupting membranes or internal structures
- Enterotoxins
- Act on the gut; cause watery diarrhea or vomiting by disturbing water and ion balance in the intestinal epithelium
- Neurotoxins
- Affect nerve cells or neuromuscular transmission
Some exotoxins can be converted to toxoids (inactivated toxins) and used as vaccines.
Endotoxins
- Components of the outer membrane of Gram‑negative bacteria, especially lipopolysaccharide (LPS)
- Released mainly when the bacteria die or multiply rapidly
- Stimulate strong inflammatory responses
In small amounts, they activate immune defenses; in high amounts, they can cause:
- High fever
- Drop in blood pressure
- Widespread activation of clotting (disseminated intravascular coagulation)
- Septic shock, a life‑threatening condition
Types and Courses of Bacterial Infections
Bacterial diseases can present in characteristic patterns:
Localized Infections
- Restricted to one body region or organ
- Examples: tonsillitis, otitis media, superficial wound infection
- Symptoms often dominated by local inflammation: redness, heat, swelling, pain, loss of function
Systemic Infections and Sepsis
- Bacteria in the blood (bacteremia) can disseminate to multiple organs
- If the immune response becomes dysregulated, sepsis develops:
- General signs: high or low body temperature, rapid pulse, rapid breathing
- Organ dysfunction: confusion, decreased urine output, low blood pressure
- Sepsis is a medical emergency and a major cause of death worldwide
Acute vs. Chronic
- Acute infections
- Rapid onset, severe symptoms, short duration (days to weeks)
- Chronic infections
- Persist for months or years
- Bacteria may “hide” in poorly accessible sites or inside cells
- Symptoms may be mild but long‑lasting and damaging (e.g., chronic lung or bone infection)
Toxin‑Mediated vs. Invasive Disease
- In some illnesses, the main harm comes from ingested toxins (e.g. preformed in food), even if few live bacteria are present.
- In others, disease is mainly due to invasive growth and destruction of tissues, with toxins playing a supporting role.
Host–Pathogen Interaction: Why Some People Get Sicker
Whether exposure to a pathogenic bacterium leads to disease depends on:
- Infectious dose
- The number of bacteria required to cause infection
- Some pathogens cause disease from very few cells; others need high numbers
- Virulence and strain differences
- Not all strains produce the same toxins or have the same virulence factors
- Host factors
- Age (very young and very old are more susceptible)
- Immune status (immunosuppression, HIV infection, chronic illnesses)
- Nutritional status
- Disruption of normal microbiota (e.g., by antibiotics), which normally suppresses pathogenic bacteria
The same bacterium can cause:
- No symptoms (asymptomatic carriage)
- Mild illness
- Severe, life‑threatening disease
Diagnosis of Bacterial Infections
Identifying the causative bacterium is important for targeted treatment, especially with antibiotics.
Common diagnostic approaches:
- Microscopic examination
- Stained smears from blood, sputum, pus, cerebrospinal fluid, etc.
- Shape (cocci, rods, spirals) and arrangement give clues
- Culture
- Growth of bacteria on specific nutrient media from clinical samples
- Allows identification and testing of antibiotic susceptibility
- Rapid tests
- Detection of bacterial antigens or nucleic acids (e.g., PCR)
- Useful for fast diagnosis when immediate therapy is needed
- Serology
- Detection of antibodies in the patient’s blood
- Useful for some chronic or systemic infections
Results guide the choice of therapy and help distinguish bacterial infections from viral or other causes.
Principles of Treatment
Antibiotics
Antibiotics are substances that kill bacteria (bactericidal) or inhibit their growth (bacteriostatic) without (ideally) harming human cells.
Typical targets:
- Cell wall synthesis
- Human cells lack cell walls; this makes a good selective target
- Protein synthesis
- Bacterial ribosomes differ structurally from human ribosomes
- DNA replication and transcription
- Bacterial enzymes can be selectively inhibited
- Metabolic pathways specific to bacteria
- For example, folic acid synthesis
Antibiotic Susceptibility and Resistance
Bacteria can be:
- Susceptible: killed or inhibited at achievable drug concentrations
- Resistant: survive despite treatment
Resistance can arise by mutation or by acquiring resistance genes from other bacteria. Mechanisms include:
- Enzymatic destruction or modification of the antibiotic
- Alteration of the antibiotic’s binding site
- Efflux pumps that expel the drug from the cell
- Reduced permeability of the cell envelope
Misuse and overuse of antibiotics (e.g., taking them for viral infections, stopping therapy too soon, use in animal farming) accelerate the development and spread of resistant bacteria, including multidrug‑resistant strains.
Supportive and Surgical Treatment
Besides antibiotics, treatment often includes:
- Fluid and electrolyte replacement (especially in diarrheal diseases)
- Oxygen therapy and stabilization of circulation (in sepsis)
- Pain reduction and fever control
- Surgical drainage of abscesses or removal of infected tissue or foreign material (e.g., infected implants)
Prevention of Bacterial Infections
Preventing infection is often easier and safer than treating established disease.
Hygiene and Public Health Measures
- Hand hygiene (washing or disinfection)
- Safe handling, cooking, and storage of food
- Clean water supply and sewage treatment
- Sterilization and disinfection of medical instruments
- Isolation of patients with highly contagious diseases in health care settings
Vaccination
For some bacterial diseases, effective vaccines are available. Many modern bacterial vaccines use:
- Inactivated whole bacteria
- Purified components (e.g., capsules)
- Toxoids (inactivated exotoxins)
Vaccination can prevent serious illnesses such as certain forms of meningitis, diphtheria, tetanus, and pertussis.
Protection of the Normal Microbiota
The body’s own microbiota acts as a barrier against pathogenic bacteria by:
- Competing for nutrients and attachment sites
- Producing inhibitory substances
Measures that help maintain a healthy microbiota:
- Restricting antibiotics to cases where they are medically necessary
- Avoiding unnecessary antiseptics and “sterilizing” products in everyday life
- Supporting balanced nutrition and overall health
Summary
- Most bacteria are harmless, but pathogenic species can invade, multiply, and damage the human body.
- Their disease‑causing ability depends on virulence factors such as adhesins, invasins, capsules, enzymes, and toxins.
- Bacterial infections range from localized inflammation to life‑threatening sepsis.
- Diagnosis relies on identifying the pathogen and its antibiotic susceptibility.
- Antibiotics target bacterial structures and processes but are threatened by increasing antibiotic resistance.
- Hygiene, vaccination, and protection of the normal microbiota are key strategies to prevent bacterial diseases and reduce their impact on human health.