BACTERIOLOGY PART-1

Gram-positive Cocci Exception

• All cocci are gram-positive except
• Mnemonic: VeNoM = G -ve cocci
• Veillonella
• Neisseria
• Moraxella
• moratt nyc valli

Gram-negative Bacilli Exception

• All bacilli are gram negative except
• Mnemonic- MAC DONALD = G +ve bacilli
• Mycobacterium
• Diphtheroids
• Corynebacterium diphtheria
• Spore containing
• Anthracis
• Clostridium species
• Gram positive filamentous
• Nocardia
• Actinomyces
• Different motility
• Listeria
• [Yersenia (G-ve bacilli)]

Gram-positive cocci

• Micrococcaceae family

Feature	Micrococcus	Staphylococcus
Shape	Tetrads	Bunch of grapes
Glucose utilization	Oxidative	Fermentative
Oxidase	Positive	Negative
Catalase	Positive	Positive

• Protein A
• Found in Staphylococcus aureus
• Binds Fc region of IgG
• ⛔ opsonization and phagocytosis
• Anticomplementary
• Antiphagocytic
• Co-agglutination

Superantigens Causing Shock

• Polyclonal activation of T cells

• Activate T cell directly → Cause shock and MOF

• MOA: [Cross-links β region of TCR to MHC class II on APCs]
• Antigen bind to MHC II → outside antigen binding site
• ↑↑ release of IL-1, IL-2, IFN-γ, and TNF-α → shock

• Features
• Bubbles = Catalase +ve
• Reaction: H₂O₂ --(catalase)--> H₂O + O₂ --(Bubbles d/t O₂ release)
• No Bubbles = Catalase negative
• Pneumococcus
• Streptococcus
• Enterococcus
• Shigella dynsentriae

• Clinical Relevance
• Chronic granulomatous disease
• NADPH oxidase deficiency
• DHR > NBT
• Recurrent infections with catalase ⊕ organisms

Catalase ⊕ organisms
(Mnemonic: SPACE Being Made Bubbly CATS)

• Staphylococcus aureus > Golden colour.

• Serratia > Red color (Prodogiosin)

• Pseudomonas > Blue-green color.
• Blue Nail syndrome

• Candida

• Enterobacteriaceae family.
• Not enterococci

• Escherichia coli.

• Bacillus anthracis

• Neisseria gonorrhoea

• Neisseria meningitidis

• Mycobacterium tuberculosis.

• Aspergillus

• Cryptococcus

• Micrococcus

Antibiotic Resistance

Types

• Enzyme mediated
• Aminoglycosides
• β-lactamase
• Chloramphenicol
• Amina () Betakk () Enzyme vach Choru () koduth

• Altered target
• MRSA
• PBP → PBP2a
• VRSA
• D-ala → D-lac
• Macrolides
• Linezolid

• Efflux pump
• Tetracyclines

β-lactamase

• BLA gene transmitted/coded via plasmids

• Enzyme by bacteria

• Bind to penicillin-binding proteins (PBPs) or transpeptidases on S. aureus

• break the structure of penicillin at β-lactam ring.
• ⇒ breakage.

• Transfer: Transduction > Conjugation

• Ex: H. influenza Resistance to Ampicillin
• d/t production of beta lactamase

β Lactamase Inhibitors

• Clavulanic acid, sulbactam and tazobactam
• ⛔ β-lactamase

• Combinations are:
• Clavulanic Acid + Amoxycillin
• Sulbactam + Ampicillin
• Tazobactam + Piperacillin

Penicillinase

• Specific β-lactamase

• Produced by Staphylococcus aureus

• Acts mainly on penicillin's

Penicillinase Resistant Penicillins

• Mnemonic: Put CONDOM over pencil to protect it
• C → Cloxacillin
• O → Oxacillin
• N → Nafcillin
• D → Dicloxacillin
• O M → Methicillin [Most resistant]

• Mnemonic: Oxa Cloxa Dicloxa + M and N

Methicillin resistance

• Resistance is due to altered Penicillin Binding Proteins.
• mecA gene
• PBP → PBP2a
• which has a lesser affinity for antibiotic binding.

• Diagnosis of methicillin resistance
• Cefoxitin disc diffusion agar/Oxacillin.
• Latex agglutination for PBP2a.
• ELISA / PCR for MecA.

• MRSA treatment
• DOC for MRSA : Vancomycin.
• All β-lactams are ineffective
• except 5th gen cephalosporins.
• 5th Gen:
• Ceftibiprole
• Ceftaroline
• (With “rol”)
• Against MRSA, VRSA
• Daptomycin

Vancomycin resistance

• Vancomycin-resistant enterococcus (VRE).

• VanA gene.

• Mechanism:
• Peptidoglycan precursor changes from
• D-ala-D-ala → D-ala-D-lactate / D-ala-D-serine.

• VRSA Drugs
• DOC for VRSA : Daptomycin.
• Lung surfactant → break down Daptomycin
• DOC for VRSA pneumonia : Linezolid.
• Linezolid has good lung penetration
• Streptogramins
• Quinpristin + Dalfopristin

• Say Mercy Mercy (MRSA)
• Valentines (Panton Valentine toxin A/w MRSA) dayil → game (γ hemolysin) played
• with Hymen (Synergohymentrophic toxin) doing PV (PV toxin)

Follicular Infections

• Affect the hair follicle

• Common cause for all follicular infections: Staphylococcus

Types:

1. Superficial Folliculitis (Bockhart's folliculitis)

• Limited to superficial follicle

• Presents: Minute, tender pustules with erythema

• Treatment: Topical antibiotics

2. Deep Folliculitis

• Infection extends deep into follicle

• Can lead to:
1. Sycosis Barbae:
• Not a fungal infection
• Bacterial, painful, pus, crusting
• Distinguish from fungal Tinea Barbae
• Tinea is itchier
2. Dissecting Cellulitis of Scalp 
• if on scalp

• Treatment: Oral antibiotics

• Mnemonic:
• Psycho Barbie (Sycosis Barbae) → With Pus
• Tiny barbie (Tinea Barbae) → Itching

3. Furuncle

• Folliculitis + perifollicular spread
• (erythema, inflammation, tenderness)

• Treatment: Oral antibiotics

Carbuncle

Definition

• Multiple small abscesses coalesce to form a large abscess

Clinical Presentation

• Most common: In diabetics

• Features: Multiple pus points

• Site: Most commonly the nape of the neck

Management

• Drainage with a cruciate incision (to drain all the abscesses)

Toxin-Mediated Infections

Staphylococcal Scalded Skin Syndrome (SSSS)/Ritter disease

• Cause: Staphylococcus (exfoliative toxin ET1)

• Affected: Newborns, infants

• Pathophysiology:
• Starts from staph infection elsewhere
• ET1 binds to Desmoglein 1
• Same as Impetigo
• Intraepidermal split at granular layer
• Oral mucosa spared

• Presentation:
• Exfoliating skin, peeling
• Fever, malaise, possible failure to thrive

• Diagnosis: Nikolsky test positive

• Treatment: Systemic antibiotics

• Mnemonic:
• Toxic staffs exfoliated (Staph → Exfoliative Toxin 1) a newborn on day 1 (desmoglin 1)

Causative Organisms Summary

• Single organism involved summary

Organism	Infections
Staph	• Folliculitis, Furuncles, Carbuncles • Bullous impetigo • SSSS
Strep	• Scarlet fever

• Mnemonic:
• Staphylococcus for follicular;
• Streptococcus for most non-follicular

Erysipelas vs. Cellulitis

• Similarities:
• Tenderness at onset
• Common sites: Legs or forearms
• Systemic symptoms: Fever, malaise
• Swelling present

• Differentiation:
• Erysipelas:
• Group A β Hemolytic Streptococci (Pyogenes)
• Clear, demarcated margin
• Affects superficial dermis & lymphatics
• Million ear sign → Ear pinna involved = Erysipelas
• Cellulitis:
• Strep or Staph
• Diffuse, not clearly demarcated
• Deep subcutaneous infection
• Affects deep dermis to subcutis

• Treatment:
• Both: Oral antibiotics, leg elevation, NSAIDs
• Cellulitis: May need longer therapy

Non-Follicular Infections

Impetigo

• Strep > Staph

Bullous Impetigo:

• Bullae (blisters) present

• Common in newborns

• Cause: Only Staphylococcus aureus → Exfoliative toxin

• Crust color: Golden

• Target: Desmoglein 1 (in skin, for bulla)

• Bullae: Superficial, hypopyon (fluid at bottom)

Non-bullous Impetigo (Impetigo Contagiosum):

• Starts as bullae, then ruptures to form crust

• Presentation: Crusted erosions

• Common in toddlers/school-aged children

• Cause:
• Streptococcus (developing nations) >
• Staphylococcus (developed nations)

• Crust color: Honey-colored

• Complication: Post-streptococcal glomerulonephritis (rare)

General Characteristics:

• Mainly on face (common)

Diagnosis:

• Gram stain (Staph & Strep: Gram-positive cocci)

Treatment:

• Mild: Topical antibiotics (Mupirocin, Fusidic acid)

• Severe: Oral antibiotics

Scarlet Fever

• Second disease

• Desquamatous rash
• Sandpaper rash

• S. Pyogenes

• Strawberry tongue

• Pastia lines

• Strawberry, Pasta in a Sandpaper
• For pus patient on second day

 

• M protein
• Helps prevent phagocytosis
• Expressed by group A Streptococci
• Sequence homology with human cardiac myosin (molecular mimicry)
• Autoimmune response in acute rheumatic fever

NOTE

• Capsular polysaccharide:
• Found in Streptococcus pneumonia
• Over 90 known serotypes.
• Identified by the Quellung reaction.
• Highly charged, hydrophilic structure
• Acts as barrier to phagocytosis
• Acts as barrier to complement-mediated lysis
• Key determinant of virulence

• Pneumococcus corneal ulcer:



• Most common cause in India.
• Can lead to "Ulcus serpens" (snake-shaped ulcer).
• Causes "Hypopyon"
• Mobile, sterile collection of pus in anterior chamber
• Mnemonic: pneumo → new lens in kollam → snake

• Myringitis bullosa
• Caused by
• Influenza Virus > Pneumococcus > Mycoplasma
• Bullous vesicles are seen on the tympanic membrane.

• Secretes IgA protease → cleaves IgA → colonize mucous membranes

• Includes
• S. Pneumonia
• H influenza
• N meningitidis
• Vaccine against ACYW (Not B)

• Spleen Clearance & Asplenia
• Capsulated bacteria are opsonized → then cleared by the spleen
• Asplenics have ↓ opsonizing ability → ↑ risk for severe infections.
• SHIN Vaccine is given in Splenectomy

• Also Common answer in
• Has Capsule
• Has Polysaccharide subunit vaccine
• Also include Typhoid vaccine
• Transformation ⇒ Griffith experiment
• Secrete IgA Protease

Also has capsule:

 

Neisseria meningitidis

• Clinical features:
• Pyogenic meningitis.

• Meningococcal vaccine:
• Protects against serotypes A, C, Y, W135.
• Does not protect against serotype B.

Neisseria Gonorrhoeae

• Clinical features:
• Ophthalmia neonatorum.
• Urethritis (most common in males).
• Cervicitis (most common in females).
• Watercan perineum (multiple discharging sinuses).

• Complications:
• Disseminated gonococcal infection (DGI).
• A/w PORB1a.

• Fitz-Hugh-Curtis syndrome
• perihepatitis
• Also seen in Chlamydia trachomatis infections.
• Trac (trachomatis ) gone (Gonorrhoeae) fits (Fitz-Hugh-Curtis syndrome varum

Modified Thayer-Martin medium

• Contains Vancomycin, Colistin, Nystatin, and Trimethoprim.

• Bilateral adrenal hemorrhage
• Waterhouse-Friedrichsen syndrome

Case scenario

• An unimmunized 8 yr old girl presented with fever, sore throat and swelling of neck.

• The examination of throat revealed the lesion shown in image 1.

• The picture seen on Albert stain of the swab taken from the lesion is shown in image 2.

• What is the probable diagnosis?

Epidemiology

• Causative organism: 
• Corynebacterium diphtheriae.
• C. diphtheriae is also Klebs-Loeffler bacillus.
• Gram positive, non-motile, aerobe and facultative anaerobe
• Secretes exotoxin
• Diphtheria toxin production depends on iron levels.
• Toxoid vaccine is produced using acid and formalin solution.

• Incubation period: <1 week.

Presentation: 

• Asymptomatic > Symptomatic (3:1).

• H/o unimmunized child.

• Universal early symptom: Sore throat.

• Fever (only in 50%), malaise, headache.

• Dysphagia, hoarseness.

• Stridor & cough.

• Bull neck appearance.

• Drooling of saliva.

• Throat examination Pseudomembrane.
• Removal causes bleeding.
• Can extend to the tonsils, uvula, soft palate, and posterior oropharynx wall.

Isolation criteria: 

• Oropharyngeal/nasopharyngeal swabs

• Diphtheria → 2 culture negative

Toxins that decrease protein synthesis (Mnemonic: DPSS)

• Diphtheria toxin

• Pseudomonas Exotoxin A

• Shiga toxin (Shigella)

• Shiga-like toxin/Verocytotoxin (E. coli)

• Mechanisms of action:
• Diphtheria toxin and Exotoxin A of Pseudomonas:
• Cause ADP ribosylation of Elongation Factor-2 (Ef2).
• ⛔ protein synthesis.
• Shiga and Shiga-like toxins:
• Directly ⛔ 60S ribosomes.

Toxin Action

Membranous Tonsillitis vs Diphtheria

Notification & isolation:

• Typhoid → Till 3 negative stool cultures.

• Diphtheria → 2 culture negative

• Cholera → Isolate patient until 2–3 negative stool cultures

Period of communicability: 

• Up to 1 month after onset.

Stains:

Culture:

• McLeod agar

• Enriched media:
• Loeffler serum slope.
• Growth in 6-8 hours.
• Best medium for early diagnosis.
• Laugh (Loeffler) early (early) to become rich (Enriched)

• Selective media:
• Potassium tellurite agar/Tinsdale media.
• Produces black colonies.
• Grows in 48 hours.
• Best medium for C. diphtheriae.
• Best () story → selected (Selective)
• When pottan (Potassium tellurite) tells () a best () story of Tins dale ()

Toxigenicity/Susceptibility tests:

• Elek's gel precipitin test:
• Double diffusion in two dimensions.
• Uses filter paper with C. diphtheria antitoxin.
• Determines toxigenicity.
• A precipitin line indicates a toxigenic strain.

• Schick test:
• Intradermal test for susceptibility.
• Redness signifies susceptibility.
• Negative reaction indicates immunity (Mnemonic: SPIN).

Bio-typing of Corynebacterium species (Mnemonic: GIM)

• Gym buddies
• Minimal guy
• eat poached egg
• no effort → no infection
• Intermediate guy
• eat frog egg,
• but dont bleed for effort
• Great guy
• Daisy gives her head
• eat starch

• Gravis: Daisy head colonies, ferments starch.

• Intermedius: Frog egg colonies, non-hemolytic.

• Mitis: Poached egg colonies, least virulent, causes minimal infection.

Carriers stages:

• Most important source of infection is chronic carriers
• Nasal > Throat

• Immunization does not prevent carrier state

• Healthy.

• Incubatory.

• Convalescent.

Chemoprophylaxis

• Given regardless of the immunization status of contacts.

Complications of Diphtheria

• Early - Myocarditis, Airway obstruction.

• Long-term - Neurological (Multiple cranial nerve palsy, Quadriplegia).

Immunization, prevention and control :

Cases

• IM Procaine Penicillin G (DOC) x 14d OR Erythromycin +

• Serial cultures +

• DAT (Diphtheria anti toxin)
• Given Post skin sensitivity test
• Mild - Mod: IM
• Severe: IV

Cases	DAT dose
Pharyngeal/ laryngeal disease	30,000 - 40,000 IU
Nasopharyngeal disease	40,000 - 60,000 IU
Extensive disease (Bullneck/Hoarseness)	80,000 - 100,000 IU

Carrier stages: 

• Antibiotic (Oral Erythromycin x 10 days).

• Serial cultures → if negative, declare as non infective

Contacts:

• Assess for signs & symptoms of disease.
• Serial culture
• Oral Erythromycin/Penicillin G (DOC) x 7 days +
• Till 2 negative culture reports
• Note: Antibiotics can be stopped only after isolation criteria is met.
• Diphtheria toxoid (DT) vaccine if
• (<7 years: DPT and > 7 years: Tdap).
• 3 doses taken + last dose <5y
• No extra vaccine,
• continue NIS.
• 3 doses taken + last dose >5y
• Single dose DT booster
• Unknown status or <3 doses
• Give DT + Complete toxoid series
• 3 primary + 2 booster doses

Corynebacterium Infections

1. Erythrasma

• Cause: Corynebacterium minutissimum

• Sites: Axilla, interdigital web, groins (most common)

• Presentation: Well-demarcated, non-itchy, mildly scaly plaques

• Diagnosis:
• Woods lamp: Coral red fluorescence (due to Corporphyrin 3)

2. Pitted Keratolysis

• Cause: Micrococcus sedentarius (a Corynebacterium)
• Mnemonic: Micro holes (Pitted) in sedentary part

• Predisposing factors: Hyperhidrosis, foot moisture (e.g., swimmers)

• Presentation: Small superficial pits on feet

Virulence factors:

• Capsule: Composed of polypeptide (polyglutamate).

• Bioterrorism agent → Pulmonary anthrax

• Anthrax toxin (Mnemonic: EPL)
• Edema factor: ↑↑ cAMP.
• Protective factor: Facilitates attachment.
• Lethal factor: Causes cell death.

Types of anthrax:

• Cutaneous anthrax
• Hide-Porters disease
• Acquired via skin-to-skin contact.
• Manifests as a malignant pustule (misnomer) with a black eschar.

• Pulmonary anthrax
• Wool sorter's disease
• Causes hemorrhagic mediastinitis.
• Most common form in bioterrorism.

• Intestinal anthrax:
• From consuming undercooked meat.
• Leads to hemorrhagic enteritis.

Diagnosis:

• Serology: Ascoli's ring thermoprecipitin test and ELISA.

• Diagnosis: McFadyean test
• Polychrome Methylene Blue
• Amorphous purplish material around bacilli
• (Capsular material)

• Microscopy: Bamboo stick or box car appearance, and Medusa head appearance.

Culture:

• Blood agar: Frosted glass appearance, comet tail appearance, and beaten egg appearance.

• Penicillin agar: String of pearls appearance.

• Gelatin stab: Inverted fir tree appearance.

• Selective media: PLET media

• Travel in a Boxcar () to anthra () with gf - give string of pearls () and a ring (ascoli’s ring) → watch comet () and eat beaten egg () → when travelling, glass frosted (frosted glass app) → car hit a tree and inverted it (inverted tree app) → hit head (medusa head) → pletho sound (PLET)

 

• Robertson Cooked Meat (RCM)
• Best medium for anaerobic growth
• Shows only anaerobic growth with meat particles
• Clostridium perfringens
• Shows pink colour + saccharolytic reaction.
• Clostridium tetanus
• Shows Black colour + proteolytic reaction

NOTE

• Involves:
• Tetanolysin
• oxygen labile
• Tetanospasmin
• Causes muscle spasms
• causes tetanus.
• Both are heat labile.

• First muscle affected is the masseter muscle
• causing lockjaw/Trismus.

Mechanism of action:

• Acts presynaptically at the inhibitory neuron terminal.

• Protease → cleaves SNARE

• Prevents release of GABA and glycine from Renshaw cells in spinal cord

• Similar to strychnine poisoning (which acts post-synaptically).

Complications:

• Spastic paralysis

• Risus sardonicus (sustained sardonic grin).

• Opisthotonus position (severe hyperextension and spasticity).

Lab diagnosis:

• Microscopy: Drumstick appearance.

• Blood agar: Swarming motility.

• Gelatin stab: Fir tree appearance.
• Tree → tetany

• DOC: Penicillin

• Ig: Human tetanus immunoglobulin 250 U

Post exposure prophylaxis:

• Clean wound: 
• Brought < 6 hours + sterile cut
• Rest all unclean wound

• Unclean wound: 
• Brought > 6 hours with
• sterile cut/unsterile cuts or
• open or
• contaminated wounds.

• Full immunisation also include Boosters

• Human tetanus immunoglobulin is
• Never indicated for a clean wound
• In unknown / > 10 yrs

• TT single dose is indicated in
• All cases > 5 yrs

Q. A 40 yr old male presents to the emergency department with a crush injury on his leg. He says he had received a last dose of Tt vaccination at the age of 33. What is the preferred treatment of choice 1n this patient?

 

• It is a type of superinfection.m

• MC bacteria involved: Clostridium difficile.

• Cause:
• Long-term use of antibiotics.
• Alters gut flora.

• Clinical features: Watery diarrhoea.

• Diagnosis:
• Toxigenic culture: Culture media: 100% sensitivity
• Cefoxitin cycloserine fructose agar (CCFA).
• Cefoxitin cysteine yeast extract agar (CCYA).
• Detection of toxins via ELISA and PCR.

• MC antimicrobials implicated are:
• 3rd Gen. Cephalosporins > Clindamycin > Ampi or Amoxycillin > FQ.

• Accordion sign — Thick edematous bowel.

• Treatment:



• DOC :
• Oral Fidaxomicin (Low chances of relapse).
• Alternative :
• Oral vancomycin
• Oral Metronidazole.
• Monoclonal Ab against toxin :
• Bezlotoxumab.