Bacteria are microscopic organisms that are found everywhere around us. Many different types of bacteria live in and on our bodies. Some bacteria are harmless or even beneficial, while other bacteria can cause serious illnesses and infections. Harmful bacteria that can cause disease are called pathogens.
How bacteria can be harmful
Pathogenic bacteria can cause many different types of infections and illnesses in humans. Some of the most common types of infections caused by harmful bacteria include:
- Respiratory infections like pneumonia, bronchitis, strep throat
- Food poisoning from contaminated food or water
- Skin infections like impetigo, cellulitis, abscesses
- Urinary tract infections
- Sexually transmitted infections like gonorrhea and chlamydia
- Bloodstream infections (sepsis)
- Gastrointestinal infections like salmonella, E. coli, cholera
- Dental infections like gingivitis and periodontitis
Bacteria can cause infections when they enter the body, often through openings like the mouth, nose, eyes, urethra, or breaks in the skin. Once inside the body, bacteria can multiply rapidly if not treated promptly with antibiotics or other antimicrobial agents.
How bacteria spread
Harmful bacteria can spread between people in many different ways:
- Airborne transmission – Bacteria contained in droplets that are expelled when coughing or sneezing and then inhaled by others. Examples are tuberculosis, pertussis, meningococcal meningitis.
- Fecal-oral transmission – Bacteria in feces that contaminate food, water, surfaces, or hands and are then ingested by others. Examples are salmonella, E. coli, cholera, shigella.
- Direct contact – Spread through direct person-to-person physical contact or contact with bodily fluids. Examples are staphylococcus, streptococcus, syphilis, gonorrhea.
- Bloodborne transmission – Bacteria transmitted through contact with infected blood. Examples are HIV, hepatitis B and C.
- Sexual contact – Spread through unprotected sexual contact. Examples are gonorrhea, chlamydia, syphilis.
- Mother to child – Spread from an infected mother to her baby during pregnancy, birth, or breastfeeding. Examples are group B strep, syphilis.
- Contaminated surfaces/food – Bacteria on contaminated surfaces, food, or water that causes infection when ingested or introduced into the body. Examples are salmonella, campylobacter, E. coli.
Proper hygiene like handwashing and safe food handling can help prevent the spread of harmful bacteria.
What kills bacteria?
There are several different ways to kill or inhibit the growth of harmful bacteria:
Antibiotics
Antibiotics are medications that are designed to kill or stop the growth of bacteria. They work by targeting structures or functions unique to bacterial cells, so they are able to kill the bacteria without harming human cells. Penicillin was the first antibiotic discovered, but today there are many different types of antibiotics that work in various ways:
- Penicillins – Interfere with cell wall synthesis
- Cephalosporins – Interfere with cell wall synthesis
- Tetracyclines – Inhibit protein synthesis
- Aminoglycosides – Disrupt bacterial protein synthesis
- Macrolides – Inhibit protein synthesis
- Quinolones – Inhibit DNA replication and transcription
- Sulfonamides – Disrupt folic acid synthesis
The specific antibiotic prescribed depends on the type of bacteria causing the infection. Antibiotics are very effective at treating most common bacterial infections. However, antibiotic resistance is a growing problem, so they must be used properly to preserve their effectiveness.
Disinfectants and antiseptics
Disinfectants and antiseptics are antimicrobial chemicals that are applied topically to surfaces or skin to kill bacteria. Common examples include:
- Alcohols – Ethanol and isopropanol damage cell membranes and denature proteins.
- Chlorine compounds – Sodium hypochlorite (bleach) and chlorhexidine react with cellular components.
- Quaternary ammonium compounds – Benzalkonium chloride disrupts membrane integrity.
- Peroxides – Hydrogen peroxide produces destructive free radicals.
- Phenols – Phenol damages cell membranes.
- Iodine solutions – Povidone-iodine causes oxidative damage.
Disinfectants are used to kill bacteria on hard, non-porous surfaces while antiseptics kill bacteria on living tissue surfaces or skin. Using the right disinfectants and antiseptics helps prevent the spread of harmful bacteria.
Heat
Heat is highly effective at killing bacteria. High temperatures denature proteins, damage cell membranes, and alter metabolic functions in bacterial cells. Some methods that use heat to kill bacteria include:
- Pasteurization – Heating food to 60-100°C to kill pathogens.
- Cooking – Thorough cooking of foods above 70°C kills pathogens.
- Boiling – Boiling water at 100°C kills bacteria in water.
- Steam sterilization – Autoclaving with pressurized steam over 120°C sterilizes medical equipment.
- Dry heat sterilization – Heating to 140-170°C sterilizes glassware and metals.
Proper cooking and pasteurization is important to ensure foods are safe for consumption and free of harmful bacteria.
Radiation
Ionizing radiation like gamma radiation, x-rays, and UV light damages bacterial DNA, leading to death or inhibited reproduction. Types of radiation used for sterilization include:
- UV germicidal irradiation – UV light damages bacterial DNA.
- Gamma irradiation – High frequency electromagnetic radiation kills bacteria and viruses on medical devices or food.
- Electron beam processing – Focused electron radiation beam sterilizes materials.
Low doses of radiation effectively kill bacteria while maintaining the qualities of the material being sterilized.
Oxidizing agents
Strong oxidizing agents like ozone (O3) and hydrogen peroxide produce free radicals that damage bacterial cell components and alter chemical structures within cells, ultimately destroying the bacteria. Uses include:
- Ozone gas – Ozone gas sanitizes water and decontaminates surfaces.
- Hydrogen peroxide vapor – Sterilizes medical equipment.
- Hydrogen peroxide washes – Antiseptic wound washes.
- Peracetic acid – Disinfects surgical instruments.
Oxidizing agents can penetrate bacterial cell walls to effectively kill a broad range of pathogens.
Alcohols
Alcohols like ethanol and isopropanol denature proteins and dissolve lipids in bacterial cell membranes and walls, which destroys their structure and leads to cell death. Some examples include:
- Ethanol – The alcohol in alcoholic hand sanitizers that kills bacteria, viruses, and fungi on hands when rubbed in.
- Isopropanol – Commonly used to disinfect skin before injections as well as disinfect surfaces.
- Benzyl alcohol – An antiseptic added to pharmaceuticals due to its bactericidal effects.
Alcohols are very effective broad-spectrum antimicrobials, active against bacteria, fungi, and viruses.
Heavy metals
Heavy metal ions like silver, copper, mercury, and zinc are toxic to bacterial cells and can damage membranes, proteins, and DNA replication. Some examples include:
- Silver nanoparticles – Extremely small silver particles have antibacterial properties and can be incorporated into medical devices and wound dressings.
- Copper surfaces – Copper alloy surfaces have intrinsic antibacterial properties and are used in hospitals to reduce infections.
- Mercury compounds – Organic mercury was historically used as antiseptic compounds but is very toxic to humans.
- Zinc oxide – Has antibacterial effects and is added to products like calamine lotion and sunscreens.
Heavy metals disrupt multiple bacterical cellular processes, though some like mercury are too hazardous for general use.
Acidic environments
Most bacteria grow best in neutral pH conditions. Highly acidic environments disrupt bacterial cell functions like transport proteins and denature enzymes and DNA. Examples include:
- Low pH foods – Pickling vegetables, yogurt, and kombucha create acidic conditions that prevent bacterial growth.
- Vinegar – Acetic acid in vinegar kills many types of bacteria.
- Lemon juice – Citric acid has antibacterial properties.
- Gastric acid – Stomach acid creates an extremely acidic environment to kill ingested pathogens.
Maintaining acidic conditions through food preparation techniques, preservatives, or natural secretions inhibits bacterial growth and survival.
Surfactants and soaps
Surfactants like detergents and soaps damage bacterial cell membranes and disrupt membrane proteins that control permeability. Common antibacterial applications include:
- Soap – Surfactants in soap lift bacteria off surfaces and disrupt cell membranes.
- Hand sanitizers – Contain surfactants that damage bacterial membranes.
- Disinfecting cleaners – Quaternary ammonium compound surfactants kill bacteria.
- Shampoos – Surfactants reduce microorganisms on the scalp.
Frequent washing with soaps and detergents helps mechanically remove bacteria from surfaces while surfactants damage cell structures.
Natural substances with antimicrobial properties
In addition to the synthetic antimicrobial agents described above, there are many naturally-occurring compounds that exhibit antibacterial properties:
Essential oils
Many essential oils like oregano oil, cinnamon oil, and tea tree oil contain plant compounds like carvacrol, cinnamaldehyde, and terpinen-4-ol that are toxic to bacterial cells. Specific mechanisms may include:
- Degradation of cell wall
- Damage to cytoplasmic membrane
- Leakage of cell contents
- Inhibition of protein, nucleic acid, fatty acid, and ATP synthesis
Essential oils are attractive antibacterial agents because they often have minimal toxicity for human cells in proper doses.
Honey
Honey contains hydrogen peroxide and organic acids that create an acidic environment unfavorable for bacterial growth. Honey also has phytochemical factors with antibacterial effects such as methylglyoxal. Some types of honey like Manuka honey have especially potent antibacterial effects.
Probiotics
Probiotics are live microorganisms that provide health benefits when consumed. Certain probiotic strains release antibacterial substances like organic acids, hydrogen peroxide, and bacteriocins that inhibit pathogen growth. They also compete with pathogens for adhesion sites and nutrients.
Cranberries
Cranberries contain flavonoids like proanthocyanidins and organic acids that impair bacterial adhesion and degrade cell walls. Regularly drinking cranberry juice helps prevent urinary tract infections by reducing bacterial adherence.
Garlic
Garlic contains allicin, ajoene, and other organosulfur compounds that exhibit broad-spectrum antibacterial activity through mechanisms like reacting with thiol groups and inhibiting enzymes in bacterial cells.
Onions
Onions contain flavonoids and thiosulfinates that disrupt bacterial cell membranes, inhibit enzymes, and prevent biofilm formation. Freshly cut or crushed onions work best as antibacterials.
Ginger
Ginger contains gingerol, shogaol, and other pungent compounds that impair bacterial growth and viability through effects on membranes, motility, and reactivity. It has antibacterial effects against foodborne pathogens and oral bacteria.
Grapefruit seed extract
Grapefruit seed extract contains bioflavonoids like hesperidin and polyphenols that disrupt bacterial membranes and metabolism. It has broad-spectrum activity against both Gram-positive and Gram-negative bacteria.
These natural antimicrobials can be consumed or applied topically to inhibit a wide range of harmful bacteria due to their multifaceted mechanisms of action.
Other factors that influence bacterial growth
In addition to direct bacterial killing, there are other factors that affect the growth and survival of bacteria:
Temperature
Most bacteria grow best at moderate temperatures between 20-45°C. Extreme heat or cold slows bacterial growth. Refrigerating perishable foods limits bacterial growth.
Water availability
Bacteria require water for critical cell processes and cannot survive in dry conditions. Dehydrating foods helps preserve them by limiting microbial growth.
Oxygen levels
While some bacteria are aerobic, requiring oxygen for growth, others are anaerobic and grow best without oxygen. Controlling exposure to air can control growth.
Nutrient availability
Bacteria need nutrients like carbohydrates, proteins, fats, vitamins, and minerals to grow. Limiting nutrient sources through preservation techniques slows growth.
Competition
Beneficial bacteria like probiotics compete with pathogens for space and nutrients. They help crowd out and inhibit growth of harmful bacteria.
Optimizing these factors to create unfavorable conditions for pathogen growth helps control bacterial infections alongside directly killing bacteria with antimicrobials.
Conclusion
Many different techniques and agents can be used to kill or inhibit the growth of harmful bacteria. Antibiotics remain the primary medical treatment, while disinfectants, antiseptics, heat, radiation, oxidizing agents, acids, and alcohols are used to kill bacteria on surfaces and prevent spread. Soaps, cleaners, and handwashing mechanically remove bacteria. Natural compounds found in foods and essential oils also exhibit antibacterial properties. Controlling external factors like temperature, moisture, nutrients, and oxygen levels helps limit growth alongside directly killing bacteria. Using the appropriate antibacterial strategies helps treat infections and prevent pathogen transmission.
