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Staph Aureus morphology and gram stain

Gram-positive cocci arranged in grapelike clusters. Distinguished from other staph by being coagulase positive

Coagulase-Negative Staphylococci morphology

Gram-positive cocci in clusters. Includes S. epidermidis and S. saprophyticus

Streptococcus pyogenes (Group A strep) morphology

Gram-positive cocci in chains. It colonizes the lower GI and GU tracts

Streptococcus agalactiae (Group B Strep) morphology

Gram-positive cocci arranged in pairs or short chains. Possesses an antiphagocytic polysaccharide capsule

Viridians group Streptococci morphology

Gram-positive cocci in chains. Colonize the oropharynx

Enterococcus spp. (E faecalis, faecium) morphology

Gram-positive cocci typically arranged in pairs and chains.

Listeria monocytogenes morphology

Small gram-positive rod, associated with GI outbreaks from contaminated food.

Bacillus species (anthracis and non-anthracis) morphology

Large, gram-positive rods. Forms spores.

Clostridium species morphology

Gram-positive, anaerobic rods. Spore-forming.

Cutibacterium acnes

Small, anaerobic gram-positive rod. Common skin colonizer often seen as a blood culture contaminant

Corynebacterium spp.

Small gram-positive rod. Most are ubiquitous skin colonizers and common blood culture contaminants.

Enterobacteriaceae morphology

Gram-negative rods that are lactose fermenters

Non-Lactose Fermenting GNRs (pseudomonas, serratia, proteus) morphology

Gram-negative rods/bacilli that is a non-lactose fermenter. Ubiquitous in moist environments

Stenotrophomonas maltophilia morphology

Gram-negative rod that is a non-lactose fermenter

Neisseria meningitidis and gonorrhoeae morphology

Gram-negative diplococci. Meningitidis is encapsulated, gonorrhoeae is not

Haemophilus influenzae

Gram-negative coccobacilli

Moraxella catarrhalis morphology

Gram-negative coccobacilli or diplococci

Acinetobacter baumannii morphology

Gram-negative coccobacilli that are non-lactose fermenters

Why do atypicals not gram stain?

They either lack a cell wall or are intracellular pathogens

Mycoplasma pneumoniae morphology

Pleomorphic and lacks a cell wall

Legionella pneumophila morphology

Gram-negative bacteria commonly found in water/moist environments

Spirochetes (borrelia burgdorferi, treponema pallidum) morphology

Thin, flexible spirals

Yeast morphology

Round or oval cells that reproduce by budding. Candida can form pseudohyphae

Molds morphology

Produce hyphae. Aspergillus has septate, mucorales has aseptate

Dimorphic fungi morphology

Exist as mold in nature and as yeast in human tissue

Inherent resistance

Inherently present in the bacteria

Acquired resistance

Not inherently present but obtained via horizontal gene transfer

Inducible resistance

Resistance that is triggered/expressed only upon exposure to an antimicrobial agent

Methicillin Resistance (MRSA) mechanism

Mediated by mec gene, encodes PBP2a. Leads to resistance against nafcillin, oxacillin, and most cephalosporins

Methicillin Susceptibility (MSSA) mechanism

Oxacillin is susceptible, meaning there is no mec gene present

Enterococci and cephalosporins

Enterococci are intrinsically resistant to all cephalosporins

Vanvomycin-Resistant Enterococci mechanism

Acquired resistance, results in significantly lower binding affinity for vancomycin

Enterococci Aminoglycoside Monotherapy

Enterococci are intrinsically resistant to aminoglycoside monotherapy. They need combo therapy with a cell-wall active agent for synergy.

ESBL (extended spectrum beta-lactamases) characteristics and DOC

Plasmid-mediated resistance that hydrolyzes penicillin, cephalosporins, and aztreonam. Does not hydrolyze carbapenems. DOC: Carbaepenem

Carbapenemases characteristics and DOC

Acquired enzymes that hydrolyze all classes of beta-lactam antimicrobials. DOC: ceftazidime/avibactam.

AmpC Beta-Lactamases mechanism

Inducible chromosomal resistance harbored by certain bacteria (enterobacter, serrtia, citrobacter). Exposure to cephalosporins triggers expression of the gene, leads to resistance to 1-3rd gen cephalosporins

Why is AmpC resistance difficult to test in the lab?

Cultures are often taken before antibiotic exposure, initial MICs may show susceptibility to 3rd gen cephalosporins before the resistance is induced

Pseudomonas aeruginosa intrisic resistance

Intrinsically resistant to 1-3rd gen cephalosporins except ceftazidime

Strep pneumonia resistance mechanism

Caused by mutations in PBPs that decrease binding affinity. Beta-lactamases have never been observed in pneumococcal straits

Resistance in Atypical Microorganisms

Inherently resistance to beta-lactams because they lack a cell wall or are intracellular pathogens

Staph aureus classic presentations

Common cause of skin and soft tissue infections, bone and joint infections, and pneumonia. Can also lead to toxic shock syndrome and cause life-threatening endocarditis or sepsis.

Coagulase-Negative Staphylococci presentation

Blood culture contaminant, true infection is associated with biofilms on prosthetic devices. S. saprophyticus is a unique CoNS associated with uncomplicated UTIs

Strep pyogenes (Group A strep) presentation

Causes strep pharyngitis, scarlet fever. Is also the flesh eating bacteria responsible for necrotizing fasciitis

Strep agalactiae (group B strep) presentation

Colonizes lower GI and GU tracts and is primarily a concern during pregnancy

Strep pneumonia and viridans group presentations

Leading cause of community acquired pneumonia, otitis media, and meningitis. Viridans group strep causes endocarditis following dental surgery.

Enterococcus spp. risk factors and infections

Opportunistic pathogens that rarely infect healthy adults. Risk factors include prolonged hospitalization and use of broad-spectrum antibiotics. Common infections include catheter assocaited bacteremia, UTIs and wound infections

E. coli clinical presentation

Most common pathogen for community-acquired UTIs and pyelonephritis. Also associated with GI outbreaks and hospital-acquired sepsis

Pseudomonas aeruginosa and stenotrophomonas maltophilia

Pseudomonas is associated with hospital-acquired pneumonia, esp in pts with cystic fibrosis or on mechanical ventilation. Stenotrophomonas is associated with ventilator associated pneumonia in the ICU.

N. meningitidis classic symptoms

High fever, headache, stiff neck. Rash appears in later stages of infection

N. gonorrhoeae and chlamydia trachomatis

N. gonorrhoeae causes urethral discharge and dysuria, can rarely cause septic arthritis. C. trachomatis is the most common STD in the US.

Listeria monocytogenes risk factors

Transmitted via contaminated food. In pregnant women, can cause miscarriage or neonatal sepsis. In elderly, often leads to meningitis or sepsis.

C. diff presentation

Nosocomial pathogen that causes colitis and diarrhea

C. perfringens presentation

Spore-forming anaerobe that causes severe, life-threatening necrotizing soft tissue infections

Legionella transmission and disease

Acquired by inhaling aerosols from contaminated water systems, causes legionnaire’s disease

Borrelia burgdorferi symptoms

Characterized by the erythema migrans rash, has bull’s eye appearance.

Treponema pallidum

3rd most common STD, diagnosed using a combo of non-treponemal and treponemal tests

Candida spp. opportunistic infections

common causes of thrush and female yeast infections characterized by white discharge

Cryptococcus neoformans and pneumocystic jirovecii

Cryptococcus is an encapsulated yeast associated with meningitis in HIV+ patients. Pneumocystis is an opportunistic fungus that causes pulmo disease in immunosuppressed or HIV patients.

Dermatophytes

Fungi that invade skin, present as athlete’s food, jock itch, or ringworm

Strep pyogenes DOC

Penicillin or amoxicillin

Strep agalactiae DOC/prophylaxis

Ampicillin or cefazolin administered during labor

MSSA DOC

Nafcillin, oxacillin, or cefazolin

HA-MRSA DOC

IV Vancomycin

CA-MRSA DOC

Clindamycin, bactrim, or doxycycline

Enterococcus spp. DOC

IV vanco

Listeria monocytogenes DOC

Ampicillin or penicillin

ESBL-producing enterobacteriaceae DOC

Carbapenems

Carbapenem-producing DOC

Ceftazidime/avibactam

AmpC beta-lactamase DOC

Cefepime or a carbapenem

Pseudomonas aeruginosa DOC

Cefepime, piperacillin/tazobactam, or a carbapenem

Stenotrophomonas maltophilia DOC

Bactrim

Acinetobacter baumannii DOC

Carbapenem

Neisseria meningitidis DOC/prophylaxis

IM ceftriaxone or PO ciprofloxacin

Borrelia burgdorferi DOC

Doxycycline

Treponema pallidum DOC

Penicillin

Pneumocystis jirovecii DOC

Bactrim

Tinea spp. DOC

Topical antifungals (clotrimazole, miconazole, butenafine)

Onychomycosis DOC

Oral terbinafine

Mucorales spp. DOC

Posaconazole or isovuconazole