Chapter 13

1. Chapter 13 Microbe-Human Interactions: Infection and Disease

2. Introduction • The human body exists in a state of dynamic equilibrium with microorganisms • The healthy individual is able to maintain a balanced coexistence with the microorganisms • This balance is disrupted on occasion allowing microorganisms to cause an infection or disease • Human-microbe interactions lead to: • microbial stabilization of body surfaces and protection from infections • maturation of host defenses and development of the immune system • invasion by microbes and their growth in sterile tissues • disease caused by microbial damage to tissues and organs

3. Contact, Colonization, Infection, Disease • Microbes that engage in mutual or commensal associations - normal (resident) flora, indigenous flora, microflora, microbiota, commensals • These microbes have colonized the body, but they remain in the outer surfaces without penetrating into sterile tissues or fluids • Infection - a condition in which microbes penetrate host defenses, invade sterile tissues and multiply - the microbes are considered infectious agents or pathogens • If an infection causes damage or disruption to tissues and organs, it is considered to be an infectious disease • Disease – any deviation from health, disruption of a tissue or organ

5. Resident Flora: The Human as a Habitat • The human body offers a seemingly endless variety of microenvironments and niches, with wide variations in temperature, pH, nutrients, moisture, and oxygen tension from one area to another • So the human body supports an abundance of microbes which includes bacteria, fungi, protozoa, viruses and arthropods • Most areas of the body in contact with the outside environment harbor resident microorganisms; large intestine has the highest numbers of bacteria • Internal organs and tissues and their fluids are generally microbe-free

6. Resident Flora: The Human as a Habitat • The vast majority of microbes that come in contact with the body are removed or destroyed by the host’s defenses long before they are able to colonize a particular area - transients • Microbes that are able to avoid the unwanted attention of the body’s defenses and become established are residents • Bacterial flora benefit host by preventing overgrowth of harmful microbes – microbial antagonism

9. Initial Colonization of the Newborn • Uterus and contents are normally sterile and remain so until just before birth • Breaking of fetal membrane exposes the infant; all subsequent handling and feeding continue to introduce what will be normal flora

11. Indigenous Flora of Specific Regions • Flora is a complex mixture of hundreds of species differing somewhat in quality and quantity from one individual to another

17. Major Factors in the Development of an Infection • Pathogens are parasitic microbes whose relationship with a host results in infection and disease • The type and severity of infection depend both on the pathogenicity of the organism and the condition of the host • There are two general groups of pathogens • true pathogens (primary pathogens) • opportunistic pathogens • Severity of the disease depends on the virulence of the pathogen; characteristic or structure that contributes to the ability of a microbe to cause disease is a virulence factor

18. True Pathogens • True pathogens (primary pathogens) – capable of causing disease in healthy persons with normal immune defenses. They are generally associated with a specific, recognizable disease • influenza virus, plague bacillus, malarial protozoan

19. Opportunistic Pathogens • Opportunistic pathogens – cause disease when the host’s defenses are compromised or when they become established in a part of the body that is not natural to them Opportunists are not considered pathogenic to a normal healthy person and do not generally possess well-developed virulence properties • Pseudomonas sp and Candida albicans

21. Course of infection • Portals of entry – characteristic route a microbe follows to enter the tissues of the body • skin - nicks, abrasions, punctures, incisions • gastrointestinal tract – food, drink, and other ingested materials • respiratory tract – oral and nasal cavities • urogenital tract – sexual, displaced organisms • transplacental • Exogenous agents originate from source outside the body • Endogenous agents already exist on or in the body (normal flora)

26. Requirement for an Infectious Dose (ID) • Minimum number of microbes required for infection to proceed • Microbes with small IDs have greater virulence • Lack of ID will not result in infection

28. Attaching to the Host • Adhesion – microbes gain a stable foothold at the portal of entry; dependent on binding between specific molecules on host and pathogen • fimbrae • flagella • adhesive slimes or capsules • cilia • suckers • hooks • barbs

31. Surviving Host Defenses • Initial response of host defenses comes from phagocytes • Antiphagocytic factors – used to avoid phagocytosis • Species of Staphylococcus and Streptococcus produce leukocidins, toxic to white blood cells • Slime layer or capsule – makes phagocytosis difficult • Ability to survive intracellular phagocytosis

32. Causing Disease • Virulence factors – traits used to invade and establish themselves in the host, also determine the degree of tissue damage that occurs – severity of disease • Exoenzymes – digest epithelial tissues and permit invasion of pathogens • Toxigenicity – capacity to produce toxins at the site of multiplication • endotoxins – lipid A of LPS of Gram-negative bacteria • exotoxins – proteins secreted by Gram-positive and Gram-negative bacteria • Antiphagocytic factors – help them to kill or avoid phagocytes; remain an irritant to host defenses

37. The Process of Infection and Disease • 4 distinct stages of clinical infections: • incubation period - time from initial contact with the infectious agent to the appearance of first symptoms; agent is multiplying but damage is insufficient to cause symptoms; several hours to several years • prodromal stage – vague feelings of discomfort; nonspecific complaints • period of invasion – multiplies at high levels, becomes well established; more specific signs and symptoms • convalescent period – as person begins to respond to the infection, symptoms decline

39. Establishment, Spread, and Pathologic Effects • Patterns of infection: • localized infection– microbes enter body and remain confined to a specific tissue • systemic infection– infection spreads to several sites and tissue fluids usually in the bloodstream • focal infection– when infectious agent breaks loose from a local infection and is carried to other tissues • mixed infection – several microbes grow simultaneously at the infection site - polymicrobial • primary infection – initial infection • secondary infection – another infection by a different microbe • acute infection – comes on rapidly, with severe but short-lived effects • chronic infections –progress and persist over a long period of time

41. Signs and Symptoms • Signs and symptoms are warning signals of disease • Sign – objective evidence of disease as noted by an observer • fever, septicemia, chest sounds, rash, leukocytosis, antibodies • Symptom – subjective evidence of disease as sensed by the patient • chills, pain, ache, nausea, itching, headache. fatigue

43. Signs and Symptoms of Inflammation • Earliest symptoms of disease result from the activation of the body defense process called inflammation • fever, pain, soreness, swelling • Signs of inflammation: • edema - accumulation of fluid • granulomas and abscesses – walled-off collections of inflammatory cells and microbes • lymphadenitis – swollen lymph nodes

44. Signs of Infection in the Blood • Changes in the number of circulating white blood cells • leukocytosis – increase in white blood cells • leukopenia – decrease in white blood cells • septicemia – microorganisms are multiplying in the blood and present in large numbers • bacteremia – small numbers of bacteria present in blood not necessarily multiplying • viremia – small number of viruses present not necessarily multiplying

45. Infections That Go Unnoticed • Asymptomatic, subclinical, or inapparent infection – although infected, the host doesn’t show any signs of disease, so person doesn’t seek medical attention

46. Portals of Exit: Vacating the Host • Pathogens depart by a specific avenue; greatly influences the dissemination of infection • respiratory and salivary – mucus, sputum, nasal drainage, saliva • skin scales • fecal exit • urogenital tract • removal of blood or bleeding

48. Persistence of Microbes and Pathologic Conditions • Apparent recovery of host does not always mean the microbe has been removed • Latency – after the initial symptoms in certain chronic diseases, the microbe can periodically become active and produce a recurrent disease; person may or may not shed it during the latent stage • Chronic carrier – person with a latent infection who sheds the infectious agent • Sequelae – long-term or permanent damage to tissues or organs

49. Reservoirs: Where Pathogens Persist • Reservoir – primary habitat of pathogen in the natural world • human or animal carrier, soil, water, plants • Source – individual or object from which an infection is actually acquired

50. Living Reservoirs • Carrier – an individual who inconspicuously shelters a pathogen and spreads it to others; may or may not have experienced disease due to the microbe • Asymptomatic carrier • incubation carriers – spread the infectious agent during the incubation period • convalescent carriers – recuperating without symptoms • chronic carrier – individual who shelters the infectious agent for a long period • Passive carrier – contaminated healthcare provider picks up pathogens and transfers them to other patients