RXPrep Infectious Diseases 2026: Ch. 22 (Background + Abx by Drug Class, 23 (Bacterial Infections)

Common pathogens for Skin/Soft Tissue

Staphylococcus aureus

Streptococcus pyogenes

Staphylococcus epidermidis

Pasteurella multocida +/- aerobic/anaerobic GNR (in diabetics)

Common pathogens for Heart/Endocarditis

Staphylococcus aureus (MRSA included)

Staphylococcus epidermidis

Streptococci

Enterococci

Concentration-dependent killing

Cmax:MIC

-aminoglycosides

-quinolones

-daptomycin

large dose, long intervals

Exposure-dependent killing

AUC:MIC

-vancomycin

-macrolides

-tetracyclines

-polymyxins

Dosing is variable

Time-dependent killing

Time>MIC

-Beta-lactams (penicillins, cephalosporins, carbapenems)

shorter dosing intervals, extended or continuous infusion

What antibiotics are used for CA-MRSA skin and soft tissue infections?

-Bactrim

-doxycycline

-clindamycin (D-test must be done prior to use)

What abx are used for severe SSTIs requiring IV tx or hospitalization? (cover MRSA and streptococci)

-vancomycin

-linezolid

-daptomycin

-ceftaroline

What abx are used for VRE E. faecalis?

-Pen G

-ampicillin

-linezolid

-daptomycin

-cystitis E. faecalis = nitrofurantoin, fosfomycin, doxycycline

What abx are used for VRE E. faecium?

-daptomycin

-linezolid

-cystitis E. faecium = nitrofurantoin, fosfomycin, doxycycline

What abx are used for atypical organisms?

-azithromycin

-doxycycline

-quinolones

What abx are used for pseudomonas aeruginosa?

-pip/tazo

-cefepime

-ceftazidime

-ceftazidime/avibactam

-ceftolozane/tazobactam

-carbapenems (except ertapenem)

-ciprofloxacin, levofloxacin

-aztreonam

-tobramycin

-polymyxin B

What abx are used for ESBL?

-carbapenems

-ceftazidime/avibactam

-ceftolozane/tazobactam

which abx are used for MSSA?

dicloxacillin, nafcillin, oxacillin

cefazolin, cephalexin, and other 1st and 2nd gen cephalosporins

amox/clav, ampicillin/sulbactam

Common pathogens for CNS/Meningitis

Streptococcus pneumoniae

Neisseria meningitidis

Haemophilus influenzae

Group B streptococcus/E. coli (young)

Listeria (young/old)

Common pathogens for Urinary Tract

E.coli, Proteus, Klebsiella

Staphylococcus saprophyticus

Enterococci

Common pathogens for Bone/Joint

Staphylococcus aureus

Staphylococcus epidermidis

Streptococci

Neisseria gonorrhoeae

GNR (in specific situations)

Common Resistant Pathogens?

Kill Each And Every Strong Pathogen

Klebsiella pneumoniae (ESBL, CRE)

Escherichia coli (ESBL, CRE)

Acinetobacter baumannii

Enterococcus faecalis, Enterococcus faecium (VRE)

Staphylococcus aereus (MRSA)

Pseudomonas aeruginosa

ESBL

extended spectrum beta-lactamase

-organisms producing ESBL are hard to kill, need to use carbapenems or newer cephalosporin/beta-lactamase inhibitors.

CRE

Carbapenem-resistant Enterobacteriaceae

-CRE treatment requires combination tx including polymyxins, ceftazidime/avibactam is also used

VRE

vancomycin resistant enterococcus

Which antibiotics carry the highest risk for C.diff infection?

broad-spectrum penicillins and cephalosporins

quinolones

carbapenems

clindamycin (Boxed warning)

DNA/RNA inhibitors

Quinolones (DNA gyrase, topoisomerase IV)

Metronidazole, tinidazole

Rifampin

Cell Membrane Inhibitors

Polymyxins

Daptomycin

Telavancin

Oritavancin

Protein Synthesis Inhibitors

Aminoglycosides

Macrolides

Tetracyclines

Clindamycin

Linezolid, tedizolid

Quinupristin/dalfopristin

Cell Wall Inhibitors

Beta lactams (penicillins, cephalosporins, carbapenems)

Monobactams (aztreonam)

Vancomycin, dalbavancin, telavancin, oritavancin

Folic Acid Synthesis Inhibitors

Sulfonamides

Trimethoprim

Dapsone

Intrinsic resistance

Not the right drug for the job - the resistance is naturally occurring.

(e.g. E. coli is resistant to Vanco because Vanco is too large to penetrate the cell wall of E. coli)

Selection pressure

Good bacteria are killed instead of bad bacteria allowing for them to multiply.

(e.g. when abx like Vanco eliminate susceptible Enterococci (healthy gut bacteria), vancomycin-resistant enterococcus (VRE) can flourish)

Acquired resistance

resistance that develops through mutation or acquisition of new genes

Enzyme inactivation

resistance from enzymes produced by bacteria breaking down the antibiotic

(e.g. the need for beta-lactamase inhibitors)

Beta-lactamase inhibitors

clavulanate

sulbactam

tazobactam

avibactam

Common pathogens for Mouth

Mouth flora

Anaerobic GNR

Viridans group Streptococci

Common pathogens for Lower Respiratory (Community)

Streptococcus pneumoniae

Haemophilus influenzae

Atypicals; Legionella, Mycoplasma, Chlamydophilia

Enteric GNR (alcoholics)

Common pathogens for Lower Respiratory (Hospital)

Staphylococcus aureus (MRSA included)

Pseudomonas aeruginosa

Acinetobacter baumannii

Enteric GNR (including ESBL, MDR)

Streptococcus pneumoniae

Common pathogens for Upper Respiratory

Streptococcus pyogenes

Streptococcus pneumoniae

Haemophilus influenzae

Moraxella catarrhalis

Gram positive staining

thick cell wall, purple

Gram negative staining

thin cell wall, pink/red

Hydrophilic antibiotics

-Beta-lactams

-Aminoglycosides

-Vancomycin

-Daptomycin

-Polymyxins

*Small Vd = poor tissue penetration

renal elimination = drug accumulation and side effects if not dose adjusted

increased Cl or Vd in sepsis = consider loading dose and aggressive doses in sepsis

poor to moderate bioavailability = not used PO or IV:PO ratio is not 1:1

Lipophilic antibiotics

quinolones

macrolides

rifampin

linezolid

tetracyclines

*large Vd = excellent tissue penetration (good for bone, lung, and brain infections)

hepatic metabolism (more DDI and hepatotoxicity)

active against atypical pathogens

Cl/Vd minimally changed in sepsis = dose adjustments not usually needed in sepsis

excellent bioavailability = IV:PO ratio is often 1:1

Beta lactam MOA

Beta-lactam ring inhibits bacterial cell wall synthesis by binding to penicillin binding proteins (PBPs) which prevents the final step of peptidoglycan synthesis of bacterial cell walls

Gram positive cocci

Staphylococcus

Streptococcus

Enterococcus

Gram positive rods (bacilli)

C. diff

Listeria monocytogenes

Gram negative cocci

Neisseria gonorrhoeae

Neisseria meningitidis

Moraxella catarrhalis

Haemophilus influenzae

Gram negative rods (bacilli)

Klebsiella pneumoniae

Legionella

Pseudomonas aeruginosa

Escherichia coli

Proteus mirabilis

Enterobacter, Citrobacter, Acinetobacter

Helicobacter pylori

Salmonella

Providencia

What bacteria do natural penicillins cover?

Streptococci

Enterococci

gram-positive anaerobes (mouth flora)

Natural penicillins have NO coverage for gram-negative or Staphylococci

What are the natural penicillins?

Penicillin VK

Penicillin G - penicillin G benzathine (Bicillin L-A) 1.2-2.4 million units x1 dose

What boxed warning does penicillin carry?

Penicillin G benzathine NOT for IV use, IM only!

Can cause cardio-respiratory arrest and death.

What bacteria do antistaphylococcal penicillins cover?

Streptococci

MSSA

antistaphylococcal do NOT have coverage against Enterococcus, gram negative, or anaerobes

What are the antistaphylococcal penicillins?

Dicloxacillin (oral capsule)

Nafcillin (injection)

Oxacillin (injection)

What infections are antistaphylococcal penicillins preferred in?

Preferred for MSSA soft tissue, bone and joint, endocarditis, and bloodstream infections

They do not require renal dose adjustments

What to do if Nafcillin causes extravasation?

central line is preferred

use cold packs and hyaluronidase injections; remember nafcillin is a vesicant !!!!!

what bacteria do aminopenicillins cover?

Streptococci

Enterococci

gram-positive anaerobes (mouth flora)

gram-negative Haemophilus, Neisseria, Proteus, and E.coli (HNPE)

note: ampicillin covers Listeria

what bacteria do aminopenicillins combined with beta-lactamase inhibitors cover?

All of the same bacteria PLUS:

-MSSA

-more resistant gram negative Haemophilus, Neisseria, Proteus, E.coli, and Klebsiella (HNPEK)

-gram negative anaerobes (B. fragilis)

What are the aminopenicillins?

-Amoxicillin - chewable

-Amoxicillin/clavulanate (Augmentin) - chewable

-Ampicillin - injection, capsule, suspension; oral is rarely used due to poor bioavailability

-Ampicillin/sulbactam (Unasyn) - injection only, ampicillin must be diluted in NS only

what bacteria do extended-spectrum penicillins combined with beta-lactamase inhibitors cover?

-Streptococci

-Enterococci

-MSSA

-gram-positive AND gram negative anaerobes (mouth flora, B. fragilis)

-gram-negative Haemophilus, Neisseria, Proteus, E.coli, Klebsiella (HNPEK) Citrobacter, Acinetobacter, Providencia, Enterobacter, Serratia (CAPES) and Pseudomonasc

What are the extended spectrum penicillins?

piperacillin/tazobactam (Zosyn)

NOTE: only penicillin active against Pseudomonas

what are the dosage forms of the natural penicillins?

Pen V potassium: PO

Pen G aqueous: IV

Penicillin G Benzathine (Bicillin L-A): IM

What are the dosage forms for antistaphylococcal penicillins?

dicloxacillin: PO

Nafcillin: IV/IM

Oxacillin: IV

what are the dosage forms for Aminopenicillins?

amoxicillin: PO

amoxicillin/clavulanate (Augmentin): PO

ampicillin: PO (empty stomach; not commonly used)); IV/IM

ampicillin/sulbactam (Unasyn): IV

what are the dosage forms of ES penicillins?

piperacillin/tazobactam (Zosyn): IV; prolonged/extended infusions (over 4 hours)

What are side effects of penicillins?

-risk of seizures when accumulation occurs from not being renal dose adjusted

-GI upset, diarrhea

-Rash including SJS/TEN, allergic rxn, anaphylaxis

-Hemolytic anemia identified with a positive Coombs test

-renal failure

-elevated LFTs

What are contraindications of penicillins?

Type 1 hypersensitivity rxn

what are the CI for Augmentin and amoxicillin 875mg strength and ER?

severe renal impairment (CrCl <30), do NOT use extended-release forms of amoxicillin or Augmentin 875 mg

what are the CI to Augmentin and Unasyn?

hx of cholestatic jaundice or hepatic dysfunction associated with previous use

what is the boxed warning for Pen G benzathine?

not for IV use! can cause cardio-respiratory arrest and death

what are the monitoring parameters for PCNs?

renal function, symptoms of anaphylaxis, CBC, and LFTs with prolonged courses

what are clinical pearls for antistaphylococcal pcns?

preferred for MSSA soft tissue, bone and joint, endocarditis, and bloodstream infxns

no renal dose adjustments

nafcillin is a vesicant -> administer through central line (use cold packs and hyaluronidase injections if extravasation occurs)

what are the clinical pearls for aminopenicillins?

ampicillin PO is rarely used due to poor bioavailability

IV ampicillin and Unasyn is preferably diluted in NS

Significant penicillin drug interaction?

1.) **Probenecid (uric acid reducer for gout) can increase the levels of beta-lactams by interfering with renal excretion.

Sometimes this is done intentionally in severe infections to increase antibiotic levels.**

2.) Beta-lactams (not nafcillin or dicloxacillin; opposite effect) can enhance anticoag effect of warfarin by inhibiting vitamin K clotting factors.

3.) Penicillins can increase concentrations of MTX

When is it ok to use penicillins in patients with a beta-lactam allergy?

Treatment of syphilis during pregnancy or in patients with poor compliance/follow up

-desensitize and treat with penicillin G benzathine

What infection is penicillin VK first-line for?

strep throat aka pharyngitis

What infections is amoxicillin first-line for? what are the doses?

-Acute otitis media (pediatric 80-90 mg/kg/day)

-Infective endocarditis prophylaxis prior to dental procedures (2 g PO x1 30-60 min prior to appt)

-H.pylori treatment

What infection is amoxicillin/clavulanate first-line for? what are the doses?

-Acute otitis media (pediatric 90 mg/kg/day)

-Bacterial sinusitis

how can you decrease the chance of diarrhea when using Augmenting?

use lowest dose of clavulanate

what are the key points for the antistaphylococcal pcns?

cover MSSA

no renal dose adjustments needed

what are key points for Zosyn?

only pcn active against pseudomonas

extended infusions (4 hours) can be used to maximize T>MIC

Which cephalosporins work best against gram negative bacteria?

Gram negative spectrum increases with each generation

what type of bacteria do cephalosporins not cover?

Enterococcus spp.

atypical organisms

What bacteria do first-generation cephalosporins cover?

gram-positive cocci (Strep and Staph only)

preferred cephalosporin for MSSA

Some activity against gram neg rods: Proteus, E.coli, and Klebsiella (PEK)

What bacteria do second-generation cephalosporins cover?

Staphylococci, S.pneumo, Haemophilus, Neisseria, Proteus, E.coli, Klebsiella (HNPEK)

cefotetan and cefoxitin - added activity against gram-negative anaerobes (B. fragilis)

What bacteria do third-generation cephalosporins cover?

1st group: ceftriaxone, cefotaxime, cefdinir (and other oral drugs):

-resistant Streptococci (S. pneumo and viridans group)

-Staphylococci (MSSA)

-gram positive anaerobes

-resistant strains of HNPEK

2nd group: ceftazidime:

-no gram positive activity

-covers Pseudomonas

What does HNPEK stand for?

Haemophilus

Neisseria

Proteus

E.coli

Klebsiella

What bacteria do fourth-generation cephalosporins cover?

cefepime: broad gram negative coverage (HNPEK, CAPES, and Pseudomonas) and gram positive (Staph and Strep)

What does CAPES stand for?

Citrobacter

Acinetobacter

Providencia

Enterobacter

Serratia

What bacteria do fifth-generation cephalosporins cover?

-resistant strains of HNPEK

-broad gram pos activity

-MRSA (ONLY beta-lactams to cover MRSA)

What bacteria do beta-lactamase inhibitor cephalosporin combos cover?

-lacks gram pos activity

-added coverage against MDR Pseudomonas and MDR gram-negative rods

-Avycaz has activity against some cRE

what bacteria do siderophore cephalosporins cover?

-PEK

-Enterobacter

-Pseudomonas

-CRE

What are the first-generation cephalosporins? what are their dosage forms?

cefazolin*: IV/IM

cephalexin (Keflex)*: PO 250-500 mg q6-12h

cefadroxil: PO

What are the second-generation cephalosporins? what are their dosage forms?

cefuroxime*: PO/IV/IM

cefotetan (Cefotan)*: IV/IM

cefaclor: PO

cefoxitin*: IV/IM

cefprozil: PO

What are the third-generation cephalosporins? what are their dosage forms?

group 1:

-cefdinir*: PO

-ceftriaxone*: IV/IM

-cefotaxime: IV/IM (not available in US)

-cefixime (Suprax): PO; chewable

-cefpodoxime: PO

group 2:

-ceftazidime (Fortaz, Tazicef)*: IV/IM

What are the fourth-generation cephalosporins? what are their dosage forms?

cefepime*: IV/IM

What are the fifth-generation cephalosporins? what are their dosage forms?

ceftaroline fosamil (Teflaro)*: IV

ceftobiprole medocaril: IV

what are the cephalosporin/Beta-lactamase inhibitor combinations? what are their dosage forms?

ceftazidime/Avibactam (Avycaz)*: IV

ceftolozane/tazobactam (Zerbaxa): IV

what are the siderophore cephalosporins? what are their dosage forms?

cefiderocol: IV

What are important drug interactions with cephalosporins?

1.) drugs that decrease stomach acid:

-cefuroxime, cefpodoxime, cefdinir should be separated by 2hrs from short-acting antacids

-H2RAs and PPIs should be avoided

2.) ceftriaxone + calcium-containing IV fluids: forms insoluble precipitates when administered in same line

-CI in neonates

-administered at diff times if IV line is flushed in adults

What are the contraindications of ceftriaxone?

ceftriaxone in hyperbillirubinemic neonates: can cause biliary sludging, kernicterus

concurrent use w/ Ca-containing IV products in neonates

What are the warnings of cephalosporins?

-Cross-sensitivity with PCN allergies due to structural similarity, <10%, highest risk is with 1st generation cephalosporins

do not pick a cephalosporin on the exam if pt has PCN allergy (unless pediatric w/ acute otitis media and mild PCN allergy)

-cefotetan and alcohol ingestion -> can cause disulfiram-like rxn

What are the side effects of cephalosporins?

-Seizures when accumulation occurs from not being renal dose adjusted*

-GI upset, diarrhea*

-Rash including SJS/TEN, allergic rxn, anaphylaxis*

-Hemolytic anemia identified with a positive Coombs test*

-myelosuppression w/ prolonged use, increased LFTs, drug fever

(similar to PCNs)

what should be monitored in cephalosporins?

renal function, signs of anaphylaxis, CBC, LFTs

When is it ok to use cephalosporins in patients with a penicillin allergy?

In pediatric patients with acute otitis media and a mild penicillin allergy