PATHOLOGY OF BACTERIAL INFECTIONS

1. PATHOLOGY OF BACTERIAL INFECTIONS

2. GRAM POSITIVE BACTERIAL INFECTIONS Common gram-positive pathogens include: Staphylococcus, Streptococcus, Enterococcus, Less common diseases caused by gram-positive rod-shaped organisms: diphtheria, listeriosis, anthrax, and nocardiosis. Clostridia, which are gram-positive, are discussed with the anaerobes. All these infections are diagnosed by culture and some special tests mentioned below.

3. Staphylococcal Infections Staphylococcus aureus are pyogenic gram-positive cocci that form clusters like bunches of grapes. These bacteria cause a myriad of skin lesions (boils, carbuncles, impetigo, and scalded-skin syndrome) as well as abscesses, sepsis, osteomyelitis, pneumonia, endocarditis, food poisoning, and toxic shock syndrome (TSS)

4. Staphylococcal Infections S. epidermidis, a species that is related to S. aureus, causes opportunistic infections in catheterized patients, patients with prosthetic cardiac valves, and drug addicts. S. saprophyticus is a common cause of urinary tract infections in young women.

5. Staphylococcal Infections S. aureus possess a multitude of virulence factors, which include surface proteins involved in adherence, secreted enzymes that degrade proteins, and secreted toxins that damage host cells

6. Staphylococcal Infections A furuncle, or boil, is a focal suppurative inflammation of the skin and subcutaneous tissue, either solitary or multiple or recurrent in successive crops. Furuncles are most frequent in moist, hairy areas, such as the face, axillae, groin, legs, and submammary folds.

7. Staphylococcal Infections Beginning in a single hair follicle, a boil develops into a growing and deepening abscess that eventually "comes to a head" by thinning and rupturing the overlying skin.

8. Staphylococcal Infections A carbuncle is a deeper suppurative infection that spreads laterally beneath the deep subcutaneous fascia and then burrows superficially to erupt in multiple adjacent skin sinuses. Carbuncles typically appear beneath the skin of the upper back and posterior neck, where fascial planes favor their spread.

9. Staphylococcal Infections Hidradenitis is a chronic suppurative infection of apocrine glands, most often in the axilla.

10. Infections of the nail bed (paronychia) or on the palmar side of the fingertips (felons) are exquisitely painful. They may follow trauma or embedded splinters and, if deep enough, destroy the bone of the terminal phalanx or detach the fingernail.

11. S. aureus lung infections usually occur in people with a hematogenous source, such as an infected thrombus, or a predisposing condition such as influenza.

12. Staphylococcal scalded-skin syndrome, also called Ritter disease, most frequently occurs in children with staphylococcal infections of the nasopharynx or skin. There is a sunburn-like rash that spreads over the entire body and evolves into fragile bullae that lead to partial or total skin loss.

13. The desquamation of the epidermis in staphylococcal scalded-skin syndrome occurs at the level of the granulosa layer, distinguishing it from toxic epidermal necrolysis, or Lyell's disease, which is secondary to drug hypersensitivity and causes desquamation at the level of the epidermal-dermal junction.

14. Staphylococcal Infections Antibiotic resistance is a growing problem in treatment of S. aureus infections. Methicillin-resistant S. aureus (MRSA) are resistant to all currently available beta-lactam cell-wall synthesis inhibitors (which include the penicillins and cephalosporins).

15. Streptococcal Infections Streptococci are gram-positive cocci that grow in pairs or chains and cause a myriad of suppurative infections of the skin, oropharynx, lungs, and heart valves.. S. pyogenes (group A) causes pharyngitis, scarlet fever, erysipelas, impetigo, rheumatic fever, TSS, and glomerulonephritis

16. S. agalactiae (group B) colonizes the female genital tract and causes sepsis and meningitis in neonates and chorioamnionitis in pregnancy. S. pneumoniae, the most important α-hemolytic streptococcus, is a common cause of community-acquired pneumonia and meningitis in adults.

17. The viridans group streptococci include several species of α-hemolytic and nonhemolytic streptococci that are normal oral flora and are also a common cause of endocarditis. S. mutans is the major cause of dental caries. Streptococcal infections are diagnosed by culture, and the rapid antigen test for pharyngitis.

18. Streptococcal infections are characterized by diffuse interstitial neutrophilic infiltrates with minimal destruction of host tissues. The skin lesions caused by streptococci (furuncles, carbuncles, and impetigo) resemble those of staphylococci .

19. Erysipelas is most common among middle-aged persons in warm climates and is caused by exotoxins from superficial infection with S. pyogenes. It is characterized by rapidly spreading erythematouscutaneous swelling that may begin on the face or, less frequently, on the body or an extremity. The rash has a sharp, well-demarcated, serpiginous border and may form a "butterfly" distribution on the face

20. On histologic examination there is a diffuse, edematous, neutrophilic inflammatory reaction in the dermis and epidermis extending into the subcutaneous tissues. Microabscesses may be formed, but tissue necrosis is usually minor.

21. Streptococcal pharyngitis, which is the major antecedent of poststreptococcal glomerulonephritis is marked by edema, epiglottic swelling, and punctate abscesses of the tonsillar crypts, sometimes accompanied by cervical lymphadenopathy. Swelling associated with severe pharyngeal infection may encroach on the airways, especially if there is peritonsillar or retropharyngeal abscess formation.

22. Scarlet fever, associated with pharyngitis caused by S. pyogenes, is most common between the ages of 3 and 15 years. It is manifested by a punctateerythematous rash that is most prominent over the trunk and inner aspects of the arms and legs. The face is also involved, but usually a small area about the mouth remains relatively unaffected to produce a circumoral pallor. The inflammation of the skin usually leads to hyperkeratosis and scaling during defervescence.

23. S. pneumoniae is an important cause of lobar pneumonia

24. Enterococcal Infections Enterococci are also gram-positive cocci that grow in chains. Enterococci are often resistant to commonly used antibiotics and are a significant cause of endocarditis and urinary tract infections.

25. Diphtheria Diphtheria is caused by Corynebacterium diphtheriae, a slender gram-positive rod with clubbed ends, that is passed from person to person through aerosols or skin exudate.

26. C. diphtheriae may be carried asymptomatically or cause illnesses ranging from skin lesions in neglected wounds of combat troops in the tropics, and a life-threatening syndrome that includes formation of a tough pharyngeal membrane and toxin-mediated damage to the heart, nerves, and other organs

27. Listeriosis Listeriamonocytogenes is a gram-positive, facultative intracellular bacillus that causes severe food-borne infections In acute human infections, L. monocytogenes evokes an exudative pattern of inflammation with numerous neutrophils.

28. The meningitis it causes is macroscopically and microscopically indistinguishable from that caused by other pyogenic bacteria The finding of gram-positive, mostly intracellular, bacilli in the CSF is virtually diagnostic. More varied lesions may be encountered in neonates and immunosuppressed adults.

29. Focal abscesses alternate with grayish or yellow nodules representing necrotic amorphous basophilic tissue debris. These can occur in any organ, including the lung, liver, spleen, and lymph nodes. In infections of longer duration, macrophages appear in large numbers, but granulomas are rare

30. Infants born with L. monocytogenes sepsis often have a papular red rash over the extremities, and listerial abscesses can be seen in the placenta.

31. Anthrax Bacillus anthracis is a large, spore-forming gram-positive rod-shaped bacterium. These bacteria are common pathogens in farm and wild animals that have contact with soil contaminated with B. anthracis spores. Anthrax spores can be ground to a fine powder, making a potent biologic weapon.

32. B. anthracis is typically acquired through exposure to animals or animal products such as wool or hides. There are three major anthrax syndromes.

33. Cutaneousanthrax, which makes up 95% of naturally occurring infections, begins as a painless, pruritic papule that develops into a vesicle within 2 days. As the vesicle enlarges, striking edema may form around it, and regional lymphadenopathy develops. After the vesicle ruptures, the remaining ulcer becomes covered with a characteristic black eschar, which dries and falls off as the person recovers. Bacteremia is rare with cutaneous anthrax.

34. Inhalational anthrax occurs when spores are inhaled. The organism is carried by phagocytes to lymph nodes where the spores germinate, and the release of toxins causes hemorrhagic mediastinitis. After a prodromal illness of 1 to 6 days characterized by fever, cough, and chest or abdominal pain, there is abrupt onset of increased fever, hypoxia, and sweating. Frequently, anthrax meningitis develops from bacteremia. Inhalational anthrax rapidly leads to shock and frequently death within 1 to 2 days.

35. Gastrointestinal anthrax is an uncommon form of this infection that is usually contracted by eating undercooked meat contaminated with B. anthracis. Initially, the person has nausea, abdominal pain, and vomiting, followed by severe, bloody diarrhea. Mortality is over 50%.

36. Anthrax lesions at any site are typified by necrosis and exudative inflammation with infiltration of neutrophils and macrophages. The presence of large, boxcar-shaped gram-positive extracellular bacteria in chains, seen histopathologically or recovered in culture, should suggest the diagnosis.

37. Inhalational anthrax causes numerous foci of hemorrhage in the mediastinum with hemorrhagic, enlarged hilar and peribronchial lymph nodes. Microscopic examination of the lungs typically shows a perihilar interstitial pneumonia with infiltration of macrophages and neutrophils and pulmonary vasculitis. Hemorrhagic lesions associated with vasculitis are also present in about half of cases. Mediastinal lymph nodes show lymphocytosis, macrophages with phagocytosed apoptotic lymphocytes, and a fibrin-rich edema

38. B. anthracis is present predominantly in the alveolar capillaries and venules and, to a lesser degree, within the alveolar space. In fatal cases, B. anthracis is evident in multiple organs (spleen, liver, intestines, kidneys, adrenal glands, and meninges).

39. Nocardia Nocardia are aerobic gram-positive bacteria that grow in distinctive branched chains. In culture, Nocardia form thin aerial filaments resembling hyphae. Despite this morphologic similarity to molds, Nocardia are true bacteria

40. Nocardia appear in tissue as slender gram-positive organisms arranged in branching filaments Irregular staining gives the filaments a beaded appearance. Nocardia stain with modified acid-fast stains (Fite-Faraco stain), unlike Actinomyces, which may appear similar on Gram stain of tissue. At any site of infection, Nocardia elicit a suppurative response with central liquefaction and surrounding granulation and fibrosis. Granulomas do not form.

41. GRAM-NEGATIVE BACTERIAL INFECTIONS

42. Neisserial Infections Neisseria are gram-negative diplococci that are flattened on the adjoining sides, giving the pair the shape of a coffee bean These aerobic bacteria have stringent nutritional requirements and grow best on enriched media such as lysed sheep's blood agar ("chocolate" agar). The two clinically significant Neisseria are N. meningitidis and N. gonorrhoeae.

43. N. meningitidis is a significant cause of bacterial meningitis, particularly among children younger than 2 years of age. The organism is a common colonizer of the oropharynx and is spread by the respiratory route. Approximately 10% of the population is colonized at any one time, and each episode of colonization lasts, on average, for several months.

44. N. gonorrhoeae is an important cause of sexually transmitted disease (STD) It is second only to C. trachomatis as a bacterial causative agent of STDs. Infection in men causes urethritis. In women, N. gonorrhoeae infection is often asymptomatic and so may go unnoticed. Untreated infection can lead to pelvic inflammatory disease, which can cause infertility or ectopic pregnancy Infection is diagnosed by PCR tests, in addition to culture.

45. Whooping Cough Whooping cough, caused by the gram-negative coccobacillus Bordetella pertussis, characterized by paroxysms of violent coughing followed by a loud inspiratory "whoop." B. pertussis vaccination, whether with killed bacteria or the newer acellular vaccine, has been effective in preventing whooping cough. The diagnosis is best made by PCR, because culture is less sensitive.

46. Bordetella bacteria cause a laryngotracheobronchitis that in severe cases features bronchial mucosal erosion, hyperemia, and copious mucopurulent exudate Unless superinfected, the lung alveoli remain open and intact. In parallel with a striking peripheral lymphocytosis (up to 90%), there is hypercellularity and enlargement of the mucosal lymph follicles and peribronchial lymph nodes.

47. Pseudomonas Infection Pseudomonas aeruginosa is an opportunistic aerobic gram-negative bacillus that is a frequent, deadly pathogen of people with cystic fibrosis, severe burns, or neutropenia. Many people with cystic fibrosis die of pulmonary failure secondary to chronic infection with P. aeruginosa. P. aeruginosa can be very resistant to antibiotics, making these infections difficult to treat. P. aeruginosa

48. Pseudomonas causes a necrotizing pneumonia that is distributed through the terminal airways in a fleur-de-lis pattern, with striking pale necrotic centers and red, hemorrhagic peripheral areas. On microscopic examination, masses of organisms cloud the tissue with a bluish haze, concentrating in the walls of blood vessels, where host cells undergo coagulative necrosis

49. This picture of gram-negative vasculitis accompanied by thrombosis and hemorrhage, although not pathognomonic, is highly suggestive of P. aeruginosa infection.

50. In skin burns, P. aeruginosa proliferates penetrating deeply into the veins and spreading hematogenously. Well-demarcated necrotic and hemorrhagic oval skin lesions, called ecthyma gangrenosum Disseminated intravascular coagulation (DIC) is a frequent complication of bacteremia.