Cryptococcus neoformans
from soil contaminated with bird (pigeon) droppings
appears as a budding yeast with a thick polysaccharide capsule.
the only pathogenic fungus that has a polysaccharide capsule. The capsule appears red on mucicarmine stain (IMPORTANT) and as a clear unstained zone with India ink.
causes meningitis in AIDS patients
C neoformans usually affects immunocompromised patients (eg, kidney transplantation patients on chronic immunosuppression). It is a neurotropic fungus that is transmitted via the respiratory route and most commonly presents with subacute or chronic meningoencephalitis. Cryptococcal lung disease can occasionally cause pneumonia-like symptoms
Aspergillus fumigatus
Allergic bronchopulmonary aspergillosis (ABPA)
in
Asthma
Cystic fibrosis
Chest imaging
Recurrent fleeting infiltrates
Bronchiectasis
Diagnosis
Positive Aspergillus skin test &/or IgE
Elevated serum IgE
Eosinophilia
It is seen in biopsy specimens as septate hyphae that form acute angle V-shaped branches.
Aspergillus mainly affects immunocompromised patients.
angioinvasive pneumonia (classic triad of fever, pleuritic chest pain, and hemoptysis)
Blastomyces dermatitidis IMPORTANT
a round yeast with broad-based budding and a thick, doubly reflective wall.
immunocompetent people
Pulmonary blastomycosis is characterized by granuloma formation.
Extrapulmonary disease (skin, bone, and genitourinary system) occurs in immunocompromised patients
Candida albicans
most commonly causes oropharyngeal, mucocutaneous, and esophageal disease. It rarely causes pneumonia. Microscopy typically shows budding yeast with pseudo-hyphae
Coccidioides immitis
causes lung disease in immunocompetent individuals
and disseminated mycosis in immunocompromised patients.
is endemic to the southwestern United States (i.e., southern and central California, Arizona, New Mexico, and western Texas
a dimorphic fungus that has a mold form (hyphae) at 25° C–30° C and an endospore form (spherules containing endospores, a unique characteristic of Coccidioides) at body temperature (37° C–40° C )
C. immitis causes lung disease, which can be asymptomatic or cause flu-like symptoms (e.g., cough, fever, and myalgia) accompanied by erythema nodosum. In total, C. immitis can present in five ways: acute pneumonia (most common)
Histoplasma capsulatum
soil contaminated with bird or bat droppings.( a history of exploring caves (exposure to bats) or cleaning bird cages or coops)
Endemic to Ohio & Mississippi River Valleys
dimorphic fungus that causes tuberculosis-like pulmonary disease.
It can also cause disseminated mycosis in immunocompromised patients.
It is found intracellularly in tissue (within macrophages), appearing as small, ovoid, and budding yeast cells.
Controlled by cell-mediated immune response (granulomas)
Diagnosis
Urine antigen testing
Biopsy with histopathology – granulomas & macrophages with intracellular ovoid/round yeast
Patients with intact immune systems usually remain asymptomatic, but a minority develop subacute pneumonia (ie, cough, fever, pleuritic chest pain, pulmonary infiltrates) with hilar and mediastinal lymphadenopathy.
In immunocompromised patients (eg, AIDS), infected macrophages can spread the organism throughout the reticuloendothelial system, resulting in widespread disseminated disease.
Rhizopus
species typically causes rhino-orbito-cerebral infection but can cause pulmonary disease,
predominantly in immunocompromised patients.
Histology typically shows broad hyphae with irregular branching and rare septation
Mycobacterium tuberculosis
grows in long, serpentine cords due to the presence of CORD FACTOR, a surface glycolipid, on the cell wall. Cord factor is a primary VIRULENCE factor of M tuberculosis; it protects the bacteria from digestion by macrophages and also leads to the formation of caseating granulomas.
Mycolic acid is the primary component of the cell wall of all Mycobacterium species and is the source of their acid-fastness (ability to retain dye in the presence of acid). Although mycolic acid is a component of cord factor.
Streptococcus pneumoniae.
is a gram-positive diplococci that exhibits partial (α) hemolysis on blood agar (green colonies) and is bile-soluble and Optochin-sensitive.
Its major virulence factor is a thick polysaccharide capsule that encases the organism and prevents phagocytosis and complement binding (IMPORTANT)
Other virulence factors of S pneumoniae include IgA protease (inactivates secretory IgA), adhesins (necessary for adhesion to epithelial cells), and pneumolysin (cytotoxin that causes pores in cell membranes and cell lysis).
Pneumococcal Vaccines 💉
- Polysaccharide Vaccine (PPSV23)
23 serotypes (capsular polysaccharides)
T-cell independent → no MHC presentation (only peptides can be presented)
Weak antibody response, no memory B cells - Conjugate Vaccine (PCV13)
13 serotypes (capsular polysaccharide + inactivated diphtheria toxin protein)
Protein allows MHC presentation → T-cell–dependent response
Strong immunogenicity → high-affinity IgG, memory B cells, mucosal IgA
Group B Streptococcus (GBS)
Key Virulence Factor:
Polysaccharide capsule
Gram-positive cocci in chains
Beta-hemolytic
Major neonatal infection cause → sepsis, pneumonia, meningitis
Acute exacerbation of chronic obstructive pulmonary disease
Precipitants:
Infectious (70%) IMPORTANT
Viral: rhinovirus, influenza, RSV
Bacterial: Haemophilus influenzae (nontypeable), Moraxella catarrhalis, Streptococcus pneumoniae
Noninfectious (30%)
For example, spontaneous sterile inflammation (underlying disease), pulmonary embolism, inhaled irritants.
secondary bacterial infections
The leading pathogens are Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. The elderly are affected most commonly,
but S aureus can cause secondary pneumonia in young, previously healthy patients. (IMPORTANT)
Pertussis (whooping cough
The phases of pertussis include:
Catarrhal phase - similar to many routine upper respiratory infections (eg, malaise, mild fever, rhinorrhea)
Paroxysmal phase - marked by severe coughing spells with the classic inspiratory whoop or post-tussive emesis/syncope
Convalescent phase - during which the cough improves
Pertussis is caused by the gram-negative coccobacillus Bordetella pertussis IMPORTANT
, which produces virulence factors that include adhesins and toxins.
Pertactin, which forms the basis of the acellular pertussis vaccine, promotes B pertussis adherence to the ciliated upper respiratory epithelium. Tracheal cytotoxin subsequently promotes local tissue destruction, resulting in cough. Pertussis toxin causes excessive adenylate cyclase activity, which prevents effective phagocytosis and allows the organism to persist in alveolar macrophages and ciliated epithelial cells, leading to a prolonged disease course.
Legionella pneumophila بتلزق
gram-negative bacillus that is facultatively intracellular
Legionellosis can be divided into Pontiac fever (an acute, flu-like, self-limited disease) and the more common Legionnaires’ disease. Legionnaires’ disease should be suspected in patients with recent exposure to contaminated water
hyponatremia (IMPORTANT)
high fever
bradycardia
neurologic symptoms (eg, confusion, headache), (GI) symptoms
Diagnosis
Culture of organism on BCYE and silver stain
Legionella urine antigen test
Cystic fibrosis
Staphylococcus aureus and Haemophilus influenzae are most commonly isolated in children,
IMPORTANT:
Pseudomonas aeruginosa, a ubiquitous gram-negative bacillus, is the leading cause of respiratory tract colonization and infection in adults with CF.
Biofilm formation – The bacteria clump together within an extracellular matrix of polysaccharides, proteins, and DNA, which prevents phagocytosis and antibiotic-mediated destruction
Conversion to a mucoid phenotype that produces an alginate polysaccharide, which strengthens biofilm structure and aids in the inhibition of opsonization and phagocytosis
Pneumocystis jirovecii.
Pneumocystis pneumonia is an AIDS-defining illness that is often seen in patients who were previously unaware of HIV infection. The underlying pathogen is an atypical fungus called Pneumocystis jirovecii.
P jirovecii cannot be cultured; therefore, definitive diagnosis requires identification of the organism in respiratory secretions.
IMPORTANT :
Methenamine silver stain is frequently used to identify the cell wall of the pathogen, which often appears as a crescent, a crushed ping-pong ball, or a circular ring around a clear center. First-line treatment with trimethoprim-sulfamethoxazole is usually curative.
Listeria monocytogenes
Contaminated ready-to-eat foods IMPORTANT
Febrile diarrhea
Invasive disease in neonates, pregnant women, elderly, immunocompromised
Can cause meningitis & sepsis
Lab Features:
Gram stain: Poor visualization (Gram-positive rod, but faint)
Light microscopy: Tumbling motility IMPORTANT
Culture: Growth from blood or CSF confirms diagnosis
Not detected by: sputum silver stain, urine antigen tests
Key Pearl:
Intracellular → escapes phagosome via listeriolysin O
Malassezia furfur
causes a cutaneous mycosis (hypopigmented skin patches).
KOH preparation of skin scrapings shows short hyphae and spores (“spaghetti and meatballs”).
virulence factors
polysaccharide capsule →STREPTOCOCCUS/H.INFLUNZA
Antigenic variation → influenza, Neisseria meningitidis
Endotoxin (LPS) → gram-negative bacteria (E. coli)
Listeriolysin O → Listeria monocytogenes (escapes phagosome)
M protein(exotoxin) → Group A Strep (S. pyogenes)
Exotoxins (unlike C. botulinum, C. tetani, C. diphtheriae)
Hyaluronidase→ Enzyme for tissue spread — seen in S. aureus, S. pyogenes, C. perfringen
NOCARDIA and Bronchopneumonia
Nocardiosis
immunosuppressive therapy, AIDS high risk
Microbiology
Gram-positive rod (beaded or branching)
Partially acid-fast (mycolic acid)
Aerobic
Epidemiology
Endemic in soil
Disease from spore inhalation or traumatic inoculation into skin
Immunocompromised or elderly patients
Clinical features
Pneumonia: similar to tuberculosis
CNS involvement: brain abscess
Cutaneous involvement
Treatment
Trimethoprim-sulfamethoxazole
Surgical drainage of abscesses
Actinomycosis
gram-positive anaerobic bacteria.
Actinomyces infection most frequently leads to the formation of cervicofacial abscesses, but systemic infection can develop anywhere in the body when the mucosa is disrupted.
Pulmonary actinomycosis is usually caused by aspiration, which often leads to lower lobe consolidation with air bronchograms
Diagnosis is made by identifying the bacteria with unique filamentous, branching patterns and the characteristic sulfur granules, which are formed by calcified mycelial fragments appear yellow;
however, hematoxylin and eosin staining gives them a basophilic (purple/blue) appearance under light microscopy.
Penicillin G is the antibiotic treatment
acid-fast stain
In the acid-fast stain for mycobacteria, the smear is first treated with an aniline dye (eg, carbolfuchsin). The dye (red color) penetrates the bacterial cell wall, where it binds with mycolic acids. The slide is then treated with hydrochloric acid and alcohol. This acid alcohol dissolves the outer cell membranes of nontuberculous bacteria, but the presence of mycolic acids prevents decolorization of mycobacteria.
cell wall
N-acetylmuramic acid and N-acetylglucosamine( are the saccharides that combine with an amino acid chain to form the peptidoglycan layer) in both Gram-positive and Gram-negative cell walls.
Teichoic acid is a molecule linked to the peptidoglycan cell wall of Gram-positive bacteria
Lipopolysaccharide (LPS) is a component of the outer cell envelope of Gram-negative bacteria
ergosterol is the sterol component of fungal cell membranes
