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A Simple Guide to Germs and Disease
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  1. A Simple Guide toGerms and Disease June 2005

  2. Microbiology (1) • Microbiology is the study of organisms not visible to the naked eye, thus requiring the use of a microscope to see them (i.e. microscopic organisms or sometimes called microbes) Smallest Prions Viruses Bacteria Fungi Protozoa Largest

  3. Microbiology (2) • What are Germs? • Germs are microorganisms that are likely to cause disease • Microorganisms are found throughout the environment. Onlya very small number are pathogenic, i.e. capable of causing disease, and not all diseases are caused by microorganisms • Some bacteria live in our gut and help to digest food.Some yeasts and bacteria are used in baking, brewingor cheese making • Bacteria are just one type of micro-organism, alongside viruses, fungi and protozoa

  4. Germs* - Family Tree Germs Bacteria Viruses Fungi Protozoa GramPositive GramNegative Enveloped Non-Enveloped Giardia Cryptosporidium SporeFormers Non-SporeFormers Influenza HIv Rotavirus Poliovirus SRSV Moulds Yeasts Salmonella E.coli Campylobacter Bacillus subtilis Bacillus cereus Clostridiumperfringens Staphylococcus Listeria Cladosporium Aspergillus Penicillium Candida *Germs = all potentially harmful micro-organisms

  5. Bacteria (1) • Bacteria are prokaryotes and have a characteristic cellular organisation. They are simple, small, unicellular organisms, varying in size between 1-20 um long • Bacteria can be classified by shape: Spherical (coccus) e.g. Staphylococcus aureus Spiral/helical e.g. Treponema Rod (bacilli) e.g Escherichia coli

  6. Bacteria (2) • Not all bacteria cause disease. Most bacterial species cannot cause disease. Many even play beneficial roles e.g. producing antibiotics. Our bodies are covered with commensal bacteria (the normal flora) • Even among bacteria that can cause disease, only a few species are always pathogenic. Many free-living bacteria or members of the normal flora are potentially pathogenic in certain types of individual (particularly the immuno-compromised), but are most of the time harmless • Consumers tend to be very aware of pathogenic bacteria such as Salmonella, Listeria and E.coli that have made headlines in recent years in connection with outbreaks of food poisoning. They often regard bacteria as modern problems that are more hazardous than traditional “germs”

  7. Bacterial Classification • The nature of the cell wall determines the classification of bacteria as either Gram-positive or Gram-negative, as determined by the Gram stain • Gram-positive bacteria: cell wall is a thick multi-layer of peptidoglycan (a mixture of sugars and amino acids) • Gram-negative bacteria: cell wall is thin and surrounded byan outer membrane (made up of lipopolysaccharides and lipoprotein) Lipoteichoicacids Outermembrane Cell wall (peptidoglycan) lipoprotein Cell membrane (lipid bilayer) Gram-positive Gram-negative

  8. Bacterial Growth (1) • Given good growing conditions, a bacterium grows slightly in size or length, new cell wall grows through the centre, and the mother cell splits into two daughter cells. If the environment is optimum, the two daughter cells may split into four in 15-30 minutes. This will continue for some time until the nutrients start to run out or conditions become unstable. Conditions are rarely optimum all of the time.

  9. Bacterial Growth (2) • Typical Growth Curve for a Bacteria Population • Lag Phasegrowth is slow whilst they become used to their new environment and nutrients • Log Phaseonce the metabolic machinery is running, they start multiplying exponentially, doublingin number every few minutes • Stationary Phaseas more and more bacteria are competing for nutrients which are decreasing, growth stops and the number of bacteria stabilises • Death Phasetoxic waste products build up, nutrients have run out and thebacteria begin to die

  10. Bacterial Growth (3) Typical bacterial growth ratein optimum conditions Bacteria divideby binary fission Typical growth curvefor a bacteria population 8.0 6.0 Log10 viable organisms/ml 4.0 and so on ... 2.0 Lag Log Stationary Death 0 Time

  11. Bacteria and Temperature 100 HIGH Campylobacter Cl botulinum A&B Bacteria die if heatedfor a sufficient time. The longer the time, the greater the destruction MinimumGrowthTemp. Staph aureus E. coli Salmonella Cl perfringens Cl botulinum E Bacillus cereus 63 Listeria 40 LOW 38 LOW HIGH Bacteria grow Bacteria Grow at slower rate Heat resistance Bacteria grow quickly • Pathogenic bacteria grow best at human body temperature 37ºC. However the majority will grow between 15-45ºC • Non-sporing cells of bacteria are killed attemperatures above 60ºC. The length of timeranges depending on the organism • Boiling kills living cells, but will not kill allbacterial spores • Fridges should be set below 5ºC. Some bacteriasuch as Listeria monocytogenes can growat refrigeration temperatures 36 15 7 Bacteria stop growing, but do not die 4 0

  12. Bacterial Food Poisoning (1) • Because harmful microorganisms are present everywhere in the environment, any food can become contaminated if not properly handled before consumption. There are several causes of food poisoning(bacteria, viruses, parasites, chemicals) • Infective bacterial food poisoninginfections occur when pathogens are ingested via contaminated food and the bacteria is established in the body, usually growing inside the intestinal tract and irritating intestines e.g. Salmonella spp. and Campylobacter jejuni. The infection may involve subsequent growth in other tissues • Toxic bacterial food poisoningsome bacteria grow in food and produce a toxin within the foodwhich is then consumed e.g. Bacillus cereus and Staphylococcusaureus. When the food is consumed viable cells of the bacteriado not need to be present. Other microorganisms in food mayproduce harmful or deadly toxins while growing in the intestinaltract e.g. Clostridium perfringens, enterotoxigenic Escherichiacoli and enterohaemorrhagic E. coli

  13. Bacterial Food Poisoning (2) • The main causes of food poisoning are: • preparing foods too far in advance • not cooking foods properly • storing foods wrongly; so bacteria grow rapidly • cross contamination of foods after cooking • infection from people handling foods due to poor hygiene

  14. Bacterial Food Poisoning (3) • Campylobacter jejunimost common cause of diarrhoea Sources: raw and undercooked poultry and meat, raw milk and untreated water • Listeria monocytogenescauses listeriosis, a serious disease for pregnant women,babies, elderly and immunocompromised individuals Sources: dairy products, soft cheeses, raw and undercooked meat, poultry and meat patés • Salmonella spp.second most common cause of foodborne illness Sources: raw and undercooked eggs,undercooked poultry and meat, dairy products, fruits and vegetables

  15. Bacterial Food Poisoning (4) • Staphylococcus aureusproduces a toxin that causes vomiting shortly after ingestingSources: cooked foods high in protein e.g. cooked meats, custard, cream cakes • E. coli O157a bacterium that can produce a deadly toxinSources: undercooked hamburger/ minced beef, contaminated cooked meatand raw milk • Shigella spp.poor hygiene causes this bacterium to be easily passed from person to personSources: salads, milk and dairy products, and unclean water

  16. Fungi (1) • Fungi are eukaryotic and exist in different growth forms in different environments. They range from small single cells such as yeasts (~5um) to large complex structuressuch as mushrooms (~5cm) • The high humidity and temperatures often found in bathrooms and kitchens are conducive to fungal growth. Mould, or mildew as it is sometimes referred, likes warmth and moderateto high humidity

  17. Fungi (2) • Fungi responsible for human disease can be dividedinto two distinct morphological forms • Yeastswhich grow as oval or spherical single cells like bacteria and multiply by budding and division • Filamentous fungimore commonly known as moulds, consist of long, branching hyphae forming a mycelium. Asexual reproduction results in formation of spores

  18. Fungal Growth Bud formation Mother Daughter Yeasts - unicellularoval or spherical cells which divide by budding and division Spore Hyphaldevelopment Branched hyphae Mycelium Filamentous fungi - branched hyphal structure often pigmented with age. Reproduce by forming large amountsof spores

  19. Why are fungi a problem? • They can be responsible for infections • They can cause an allergic response • Fungi cause discolouration and deterioration of household surfaces giving the characteristic blackening of walls, tile grouting, plaster and around window frames • Can cause unpleasant odours which are difficult to disguise. Fungi such as Aspergillus, Penicillium, and Alternata have been found to produce volatile organic compounds • Numerous fungi have been found to produce volatile organic compounds (VOC) and it has been suggested that these affect the health of persons living in mouldy houses. Symptoms include headache, eye, nose and throat irritation and fatigue • VOC’s have been implicated in health issues relating to sick building syndrome • Spores of toxigenic fungi contain mycotoxins (toxic secondary metabolites). Many fungal spores are small enough to reach the alveoli in the lungs. The mycotoxins may be transported via the blood from the lung to other body sites, possibly causing other effects e.g. skin lesions

  20. What infections do fungi cause? • Fungi cause 2 types of infection: • Superficial mycoses • fungus grows at the body surface in skin, hair and nails • spread by direct contact, highly contagious and easily spreadto others • e.g. Candida albicans (thrush and nappy rash) • e.g. dermatophyte fungi: Epidermophyton, Microsporum and Trichophyton (athlete’s foot) • Deep mycoses • involve internal organs, usually life-threatening • rare except in immunocompromised people • caused by opportunistic fungi (e.g. Aspergillus) • acquired by inhalation of spores or by entry through wounds • some part of the normal body flora and are harmless unless the body’s defences are compromised in some way (e.g. Candida)

  21. Fungi are also a source of allergens • Allergic rhinitis (hayfever) • Bronchitis • Asthma • Extrinsic allergic alveolitis • Almost all microbial allergens are fungal in origin, with the major ones being Penicillium, Aspergillus, Cladosporium and Alternaria • Asthma can be initiated and provoked by allergens commonly encountered in the general environment, for example pollens (trees, grass), house dust mites and moulds, e.g. Alternaria alternata, Cladosporium herbatum and Aspergillus fumigatus

  22. Fungi in the Home • What evidence is there for occurrence of fungal contamination in the home? • There have been a number of studies investigating fungi in the home. Fungi isolated include: • Penicillium spp. • Cladosporium spp. • Aspergillus spp. • Mycelia sterilia • Rhodotorula (pink yeasts) • Sporobolomyces (yeasts) • Fungi isolated from the air was similar in homes surveyed in Scotland, Germany, Finland, North America, Taiwan, Canada and Australia

  23. Viruses • What is a virus? • It is not a bacterium • It is very small, typically 0.1um • It is not a true cell • It is not an independently living organism. It must be insidea living cell to replicate • They have genetic material but lack cell membranes, cytoplasm and machinery • The virus uses the infected cell's machinery and enzymes to generate virus parts which are later assembled into new virus particles which leave the cell to infect other cells • It can infect bacteria, fungi, plants, animals and man • It may remain viable for long time, even in dry conditions • It can survive but does not grow in food

  24. Structure of Viruses • Classification based on characteristics such as genetic material present (RNA or DNA), symmetry of virus particle and presence or absence of an envelope • Structurally there are 2 distinct types of virus:Enveloped and Non-Enveloped Non-enveloped Enveloped Protein/lipid coat Envelope Proteincoat Nucleicacid Polymerase e.g. poliovirus adenovirus rotavirus e.g. hepatitis B e.g. herpes simplex HIV Flu

  25. What infections do viruses cause? (1) • Nearly 1000 different types of viruses are known to infect humans and it has been estimated that they account for approx. 60% of human infections. The young, elderly and immuno-compromised are particularly at risk from virus infections • Many viral diseases are well known and easily identified: • from colds and flu to chicken pox and measles • from mumps and herpes to polio, hepatitis and HIV

  26. What infections do viruses cause? (2) • From the ‘Home Hygiene’ point of view, we are mainly concerned with virus infections relating to poor personal or surface hygiene, and are thus preventable • Gastrointestinal infections • Gastroenteritis • Diarrhoea • Infectious intestinal disease • Respiratory infections • Flu • Common cold • Bronchitis

  27. Viruses causing Gastrointestinal Infections • Rotavirus • Most important viral cause of diarrhoea in children worldwide • Infects virtually all children 3-5 years old in developed and developing countries • Can be asymptomatic excreter • Small Round Structured Virus (SRSV) • Play a predominant part in epidemic viral gastroenteritis • Main symptom is projectile vomiting generating aerosols and virus can be transmitted via aerosols, can contaminate work surfaces with potential for subsequent transfer to food • Astroviruses and Adenoviruses • Have been associated with gastroenteritis outbreaks in schools, nursing homes, day-care centres, children’s hospital wards • Hepatitis A • Viral hepatitis is most common of the food associated virus diseases,most common vehicle is shellfish • Easily spread in areas where good personal hygiene is not observed. • Outbreaks occur in day-care centres, hospitals, nurseries and schools • Outbreaks may lead to secondary cases in the general community

  28. Viruses causing Respiratory Infections • Viral respiratory infections are common in winter when people are crowded together indoors for longer periods of time, air in ill-ventilated rooms is more humid, favouring survival of suspended enveloped viruses • Rhinoviruses and coronavirusestogether cause more than 50% of common colds can also cause acute bronchitis • Influenza virusesmain cause of flu, can cause acute bronchitis • Parainfluenza virusescause of pneumonia in children • Respiratory Syncytial Virus (RSV)most important cause of bronchiolitis and pneumonia in infants major cause of hospital-acquired illness on neonatal wards • Adenoviruscommon cause of sore throats and acute bronchitis

  29. Virus Transmission (1) • Presence or absence of an envelope is the major structural determinant for mode of transmission • Non-enveloped virusesmost can withstand the acidic environment of the stomachor detergent-like bile of the intestines. Transmitted by respiratory and faecal-oral routes e.g. rotaviruses, SRSV • Enveloped virusesmore fragile, require an intact envelope for infectivity, sensitive to acids and detergents, so not spread via faecal-oral route; spread in respiratory droplets, blood, saliva. e.g. influenza

  30. Virus Transmission (2) • Activities of the infected host may increasethe efficiency of transmission • Coughing and sneezingviruses that are better at increasing fluid secretions or irritating respiratory lining will induce more coughing and sneezing increasing efficiency of shedding and transmission • Diarrhoea eliminates infection more rapidly but is great for contamination of the environment and therefore will spread the microbes • Projectile vomiting generates aerosols, virus can be transmitted via aerosols, can contaminate work surfaces with potential for subsequent transfer to food

  31. Protozoa (1) • Protozoa are single-celled eukaryotes • Live in water or moist places or other organisms as parasites • Have a resistant transmissable cyst stage • Transmission to humans via insect bite or accidental ingestion of infective stages

  32. Protozoa (2) • Cryptosporidium spp. infects the intestines causing Cryptosporidiosis. Large outbreaks are associated with contaminated water or treatment deficiency of water supplies • Giardia lambliacauses giardiasis, an infection of the small intestine, spread via contaminated food and water and by direct person-to-person contact. Ingestion of one or more cysts may cause disease • Entamoeba histolyticaan amoeba infecting the large intestine, causes diarrhoea/dysentery, spread via contaminated water or food • Toxoplasma gondiicauses toxoplasmosis, a very severe disease that can produce central nervous system disorders

  33. Hygiene Hazards In The Home • Micro-organisms are introduced continually into the home via: people, food, pets, water, insects, air. Wet sites such as sinks, toilets, dishcloths and facecloths can also support microbial growth and themselves become reservoirs of potentially hazardous microorganisms • Many organisms occurring in the home are of little consequence.Some organisms have the potential to cause infectious disease • Some people carry highly pathogenic organisms without themselves being affected - other family members unaware of hazard • Hands, and a whole range of food and hand contact surfaces,combine to provide transfer routes that can move pathogens around the home and ultimately into the body producing infection • Ingestion of: • microorganisms - bacteria,viruses, protozoa • toxins - bacterial, fungal • Inhalation of: • toxins • allergens/spores • microorganisms

  34. Risk of health hazard from