LECTURE 6 (Gram -ve bacteria) Flashcards
Describe Neisseria
- “kidney bean” appearance
- Aerobic, gram -ve diplococci, non-motile, non-spore forming bacteria
- Normal inhabitants of the human respiratory tract
- Rarely cause disease
What are the similarities and differences between Neisseria gonorrhoeae and Neisseria meningitidis?
SIMILARITIES:
- pathogens for humans
- typically found associated with/inside polymorphonuclear cells
- both require aerobic atmosphere with added CO2 and enriched medium for optimal growth
DIFFERENCES:
- Meningococci have POLYSACCHARIDE CAPSULES, gonococci do not
- Meningococci rarely have plasmids, most gonococci do
Describe the Neisseria outer membrane structure
- Composed of Lipid A, Core and lacks O-antigen
- Outer membrane differs from other gram -ve bacteria
[polysaccharide chains are shorter lacking the variable O-antigen. Short chain is called LIPOOLIGOSACCHARIDE (LOS)] - Has the same endotoxic power as other gram -ve bacteria (despite lacking O-antigen)
- Pili, OMPs and LOS are antigenic
Why is the Oxidase test a key test for identifying Neisseriae?
Neisseriae produce oxidase and give positive oxidase reactions
LAB CORRELATION: When bacteria are spotted on a filter paper soaked with oxidase, the Neisseriae rapidly turn dark purple
Describe the growth characteristics of Neisseria Gonorrhea?
- Grows well on chocolate agar + on specialised medium enriched to ensure its growth + require CO2 supplementation
- Small, smooth, non-pigmented colonies appear after 18-24 hrs + are well developed after 48 hrs
- Possess numerous pili
Do Gonococci have any capsules? (YES/NO)
NO
Describe the antigenic structure of Gonococci
- PILI (Fimbriae) = hairlike appendages that enhance attachment to host cells + resistance to phagocytosis
- POR = form pores through cell membrane
- OPA PROTEINS = attachment to host cell receptors
- RMP PROTEINS = associates with por in the formation of pores in the cell surface
- LIPOOLIGOSACCHARIDE = lack O-antigen + cause mucosal cell death
What is Gonorrhea?
A sexually transmitted disease caused by Neisseria Gonorrhea
EPIDEMIOLOGY:
- highest rates in women 15-19 years old + men 20-24 years old
- major reservoir is asymptomatic patients
- newborns can be infected during birth
PATHOGENESIS:
- Not normal inhabitants of respiratory + genital flora -> PILI and OPA proteins allow initial attachment of bacteria to receptors on epithelial cells -> Gonococci attach mucous membranes of Genitourinary tract, eye, rectum and throat producing ACUTE SUPPURATION (discharge of pus) leading to tissue invasion
- Organism’s IgA protease can hydrolyse secretory IgA -> block attachment to mucosa
- Porin A in cell wall inactivates the C3b component of complement -> “serum resistance” (resistant to antibodies + complement)
SYMPTOMS:
WOMEN:
- Painful/burning sensation when urinating
- Vaginal bleeding
- Purulent vaginal discharge
MEN:
- Burning sensation during urination
- A white, yellow or green discharge from the penis
- Painful/swollen testicles
COMPLICATIONS:
- In women, ascending infection of uterine tubes (SALPINGITIS/PID) -> STERILITY (unable to have kids)
- Disseminated gonococcal infections (DGI) [most common cause of septic arthritis in sexually active adults]
LAB DIAGNOSIS:
- Specimens [pus + secretions taken from urethra, cervix, rectum, conjunctiva, throat or synovial fluid]
- Smear [reveal diplococci within pus cells]
- Culture
- Nucleic acid amplification test [important for screening population]
TREATMENT:
- CEFTRIAXONE (250mg) intramuscularly as a single dose or 400mg of oral CEFIXIME as a single dose
- ADDITIONAL THERAPY: 1g AZITHROMYCIN orally in a single dose with 100mg of DOXYCYCLINE orally twice daily for 7 days
[patients who do not complete course of treatment have risk of continued transmission + selection of resistant strains]
What is Disseminated gonococcal infections (DGI)?
Occurs when the sexually transmitted pathogen Neisseria gonorrhoeae invades the bloodstream and spreads to distant sites in the body
COMMON MANIFESTATIONS:
- Arthritis
- Tenosynovitis
- Pustules in the skin
It is the most common cause of septic arthritis in sexually active adults
What is Gonococcal Ophthalmia Neonatorum?
An infection of the eye in newborns when a neonate is infected with genital gonorrhoea during passage through a birth canal
PATHOGENESIS:
Initial conjunctivitis rapidly progresses + if left untreated results in blindness
PREVENTION:
Instillation of tetracycline, erythromycin or silver nitrate into the conjunctival sac of newborns
Why are repeated gonoccal infections common?
Since protective immunity to reinfection does not appear to develop as part of the disease process due to the antigenic variety of gonococci
Describe Neisseria Meningitidis
- Produce medium-sized smooth colonies on blood agar after overnight incubation
- Most important disease-producing serogroups are A, B, C, W-135 and Y
Describe the Antigenic structure of Meningococci
- Outer membrane + LOS = play major roles in organism virulence
- Two porin proteins (POR A and POR B) = control nutrient diffusion into organism + interact with host cells
- OPA proteins = attachment to host cell receptors
- Lipid A = toxic effects
What are the three virulence factors that Meningococci have?
- Polysaccharide capsule = enables organism to resist phagocytosis by polymorphonuclear leukocytes (PMNs)
- Endotoxin = causes fever, shock
- IgA protease = cleaves secretory IgA -> helps bacteria attach to membranes of upper respiratory tract
What is Meningitis?
An infection and inflammation of the fluid and membranes surrounding the brain and spinal cord caused by Neisseria Meningitidis
EPIDEMIOLOGY:
- Transmitted by airborne droplets/direct contact
- Carriers are asymptomatic
- Group A meningococci are most likely to cause epidemics + Group B meningococci cause many cases in developed countries since not present in the vaccine
PATHOGENESIS:
- Exclusively human parasite
- Meningococcal pili attach to surface proteins in NASOPHARYNGEAL epithelium -> organism can enter bloodstream + spread to meninges, joints or be disseminated throughout the body (meningococcemia)
SYMPTOMS:
- Sudden fever
- Purplish rash
- Headache + Stiff neck
- Nausea + Vomiting
- Increased sensitivity to light
- Confusion
- KERNIG’S SIGN [severe stiffness of hamstrings causes inability to straighten the leg when the hip is flexed to 90 degrees]
- BRUDZINKI’S SIGN [severe neck stiffness causes a patient’s hips and knees to flex when the neck is flexed]
COMPLICATIONS:
- Acute purulent meningitis
- Fulminant meningococcemia [endotoxin-induced sepsis + coagulopathy]
LAB DIAGNOSIS:
- Specimens = blood + spinal fluid for culture [lumbar puncture for spinal tap]
- Smear = shows neisseriae within polymorphonuclear leukocytes or extracellularly
- Culture = blood specimens are cultured without polyanethol sultanate + CSF is cultured on chocolate agar at 37 degrees with 5% CO2
TREATMENT:
- Penicillin G
- If allergic to penicillin, CEFOTAXIME or CEFTRIAXONE
PREVENTION:
- Vaccines containing capsular polysaccharide groups A, C, Y and W-135 at age 11 with boosters at 16
- Vaccine at 9 months for anyone at high risk of meningococcal disease
What are Enterobacteriaceae and what four metabolic processes do they have in common?
Large, heterogenous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals
COMMON METABOLIC PROCESSES:
- All facultative anaerobes
- All ferment glucose
- None have cytochrome oxidase (OXIDASE-NEGATIVE)
- Reduce nitrates to nitrites as part of their energy-generating processes
Describe Enterobacteriaeceae
Short, gram-negative, oxidase-negative rods with rounded ends that contain flagella showing motility of the agar surface called “swarming”
PATHOGENESIS:
contain endotoxin in their cell walls
ANTIGENS:
- Contain “O antigen”
- H antigen is on the flagellar protein [only flagellated organisms have H antigens]
- Capsular/K polysaccharide antigen is prominent is heavily encapsulated organisms
LAB DIAGNOSIS:
- Hugh and Leifson agar (glucose test) = a +ve test turns it from green to yellow
- Triple sugar iron agar (TSI) = differentiate gram -ve rods due to ability to ferment glucose, lactose and produce hydrogen sulphide
- MacConkey’s agar = non-lactose fermenters form colourless colonies whereas lactose fermenters form coloured colonies
What is Escherichia Coli?
The most common cause of urinary tract infection and gram -ve sepsis and is associated with “traveller’s diarrhoea”
Describe Escherichia Coli
Bacteria that normally live in intestines of people and animals, but some are pathogenic
MORPHOLOGY:
- Gram -ve + motile
- Facultative anaerobe
- Non-sporulating
- Ferments lactose (distinguishes from Shigella and Salmonella)
ANTIGENS:
three antigens used for identification = O antigen (cell wall antigen), H antigen (flagellar antigen) and K antigen (capsular antigen)
SYMPTOMS:
- Begins with stomach cramps and watery diarrhoea which turns bloody within a few days
- Vomiting
- Nausea + Headaches
- Little/no fever
DIAGNOSIS:
- Physical examination
- Stool analysis
- Toxin detection (PCR)
- Blood test (CBC)
- Creatinine (kidney function)
TREATMENT:
- Mild: rest, fluid intake, rarely antibiotics
- In ICU: IV fluids + electrolytes, TMP-SMX or FLUOROQUINOLONES
What are the virulence factors of Escherichia coli?
- Mucosal interaction
[mucosal adherence with pili -> ability to invade intestinal epithelial cells] - Exotoxin production
[pore-forming cytotoxin, cytotoxic necrotising factor (CNF), Heat-labile and stable toxin, Shia-like toxin] - Endotoxin
[lipid A portion of LPS] - Iron-binding siderophore
[obtains iron from human transferrin or lactoferrin]
Describe the MOA of Stx (Shiga) toxin
1) The A-B toxin binds to the cytoplasmic membrane, enters in an endocytotic vacuole and enters the Golgi network
2) Exiting to the cytoplasm, it combines at ribosome sites involved with tRNA binding
3) Interference with protein synthesis
Why are newborns highly susceptible to sepsis?
Because they lack IgM antibodies
Describe how E. coli causes UTIs
Pili virulence factor allows E. coli to travel up the urethra and infect the bladder (CYSTITIS) + move up to infect kidney (PYELONEPHRITIS) + usually occurs in catheterised patients
SYMPTOMS:
- burning during urination (dysuria)
- frequent urination
- feeling of fullness over the bladder
ADDITIONAL INFO: E. coli with enhanced potential to produce UTI are called UROPATHOGENIC E. COLI (UPEC)
What are the 5 different types of E. coli?
- STEC/EHEC (food poisoning)
- ETEC (traveler’s diarrhoea)
- EPEC
- EAEC (watery diarrhoea with blood and mucous)
- EIEC (profuse diarrhoea + high fever)
Describe Shiga toxin-producing E. coli (STEC)/Enterohemorrhagic bacteria E. coli (EHEC)
Bacteria that comes from uncooked meat + raw milk and fruit + vegetables that enter the human stomach and begin producing toxins that cause serious illnesses
INCUBATION PERIOD:
3-8 days
SYMPTOMS:
- Stomach muscle spasms
- Diarrhoea
- Fever
- Vomiting
COMPLICATIONS:
- Haemolytic uremic syndrome (HUS)
- Acute renal failure
- Anemia
- Thrombocytopenia
Describe Enterotoxic E. coli
It is the most important cause of “traveller’s diarrhoea” and endemic diarrhoeas in developing countries
TRANSMISSION:
- consumption of food and water contaminated by infected human/carriers
- uncooked foods such as salads/marinated meats
- direct person-to-person is UNUSUAL (since infecting dose is high)
PATHOGENESIS:
- due to HEAT-LABILE ENTEROTOXIN LT and HEAT STABLE TOXINS STa and STb
- initially attach to small intestine enterocytes using BUNDLE-FORMING PILI (Bfp) to form clustered microcolonies on enterocyte cell surface -> lesion progresses with degeneration of brush border, loss of microvilli and changes in cell morphology [ATTACHING AND EFFACING (A/E) LESION]
- not toxin mediated, symptoms result from invasive damages to intestinal walls
SYMPTOMS:
- watery diarrhoea with mucous but no blood
- deformation/destruction of microvilli
Describe Salmonella
Gram-negative rods that do not ferment lactose but produce H2S. Salmonella have two serotypes: Typhoidal (causes typhoid + paratyphoid fever) and Non-typhoidal (causes Gastroenteritis + food poisoning)
THREE TYPES OF SALMONELLA INFECTIONS:
- Enterocolitis [invasion of epithelial and sub-epithelial tissue of small and large intestines + causes nausea, vomiting, headache, profuse diarrhoea, low grade fever]
- Enteric fevers [life-threatening, spread via oral transmission of contaminated food, water, toilets]
- Bacteria with focal lesions [commonly associated with S serotype choleraesuis + early invasion of bloodstream + intestinal manifestations absent + blood culture results +ve]
What are the four serotypes of salmonella that cause enteric fever?
- Salmonella Paratyphi A (serogroup A)
- Salmonella Paratyphi B (serogroup B)
- Salmonella Choleraesuis (serogroup C1)
- S Typhi (serogroup D)
What are the most common salmonella serotypes?
Enteritidis & Typhimurium
Describe Enteric (Typhoid) fever
Fever caused by Salmonella
PATHOGENESIS:
Ingestion of contaminated food or water -> Salmonella invade the small intestine + enter the bloodstream temporarily -> bacteria carried by white blood cells in liver, spleen and bone marrow where they multiply + re-enter bloodstream
INCUBATION PERIOD:
10-14 days
SYMPTOMS:
- Fever
- Red skin rash
- Hepatosplenomegaly
- Ulcers in the intestine
- Hyperplasia + necrosis of lymphoid tissue
- White coating of tongue
- WBC normal/low
- Headache, stomach pain, diarrhoea
DIAGNOSIS:
- Blood, stool + urine test
- WIDAL test (demonstration of Salmonella antibodies against O and H antigens)
- Differential medium culture
- Nucleic acid amplification test
TREATMENT:
- Ampicillin
- Trimethoprim-sulfamethoxazole
- A third generation Cephalosporin
What is Shigella?
A gram-negative, facultative anaerobic, non-spore-forming, non-motile (no flagellar (H) antigens), rod-shaped bacteria
LAB FINDINGS:
- Does not ferment lactose (unlike E. coli)
- Does not produce H2S (unlike Salmonella)
EPIDEMIOLOGY:
- Only a human disease
- Transmitted via the focal-oral route
- Invade intestinal epithelial cells + release Shiga toxin which causes cell destruction
VIRULENCE FACTORS:
- Diarrhoea-inducing enterotoxin
- Type III secretion system = causes macrophage death
- Shiga-toxin stops protein synthesis in the host
PATHOGENESIS:
1) Shigella are taken up by M cells in the large intestine by endocytosis
2) Bacteria are quickly released from endosomes, leaving shigella free in the cytoplasm + bacteria multiply and enter the inferior and lateral aspects of the epithelial cells
3) Actin filaments form a tail + push bacteria to next cells where they multiply + macrophages that take up shigella are killed + release organism
4) Infected cells die and slough off + acute inflammatory response occurs with bleeding and abscess formation
INCUBATION PERIOD:
1-2 days
SYMPTOMS:
- Dysentery (GI disease)
- Abdominal pain
- Fever
- Watery + bloody diarrhoea
- Tenesmus (rectal spasm)
DIAGNOSIS:
- Specimen
- Culture
- Nucleic acid amplification test
TREATMENT:
- Ciprofloxacin
- Ampicillin
- Doxycycline
- Trimethoprim-sulfamethoxazole
- Azithromycin
PREVENTION:
- Sanitary control of water, food, sewage
- Isolation of patients + antibiotic treatment of infected individuals
- Detection of subclinical cases + carriers
What is Klebsiella?
Non-motile, rod-shaped, gram-negative bacteria with a prominent polysaccharide capsule
ANTIGENS:
- Lipopolysaccharide (O antigen)
- Capsular polysaccharide (K antigen)
CLINICAL CORRELATION:
- responsible for hospital-acquired infections
- high level of antibiotic resistance (resistant to penicillins and cephalosporins)
TRANSMISSION:
- person-to-person contact
- surfaces
SYNDROMES:
- UTI
- Pneumonia
- Diarrhoea
- Wound infection
What are Proteus?
A part of enterobacteriaceae family of gram-negative “swarming” bacteria and is commonly found in the human intestinal tract as part of the normal human intestinal flora
MORPHOLOGY
- Gram -ve
- Motile (has flagella)
- Urease +ve
PATHOGENICITY:
- Urease
[Urease hydrolyses urea to form ammonia which raises pH + form alkaline urine -> encourages formation of stones called “STRUVITE” composed of magnesium ammonium phosphate -> stones obstruct urine flow, damage urinary epithelium + trap bacteria within stone for recurrent infection}
- Motility
- Endotoxin
DISEASES:
- UTI
- Kidney stones
DIAGNOSIS:
- Culture
TREATMENT:
- Penicillins (for Proteus Mirabilis)
- Aminoglycosides + Cephalosporins (for P. Vulgaris and P. Panneri)
- Keeping urine at a low pH
