Ahmed D (Pathogenicity & STIs) Flashcards
What are pathogens?
Organisms that have the potential to cause disease.
The different types of pathogens and the severity of the disease that they cause are very diverse.
- Bacteria
- Viruses
- Fungi
- Parasites
What is virulence?
What is virulence?
The degree of pathogenicity of a particular organism
Pathogenicity
Three characteristics:
- Invasiveness
- Infectivity
- Pathogenic potential
- Toxigenicity (ability for virus to withstand toxic chemicals)
Determinants of pathogenicity
A pathogen must successfully achieve these steps or stages of pathogenesis to cause diseases
- Ability to be transported to the host
- Adhere to, colonise, or invade the host
- Multiply in the host
- Evade host defences
- Possess mechanical, chemical, or molecular ability to damage the host
Transmission
Direct contact
- host to host
Indirect contact transmission
- contamination of inanimate objects
Droplet transmission
- coughing, sneezing
Food and water born transmission
- salmonella, cholera, hep A
Vector transmission
- mosquitos (malaria) and ticks (Lyme disease)
Route of entry for invading pathogen
Skin- breaks protective barrier, bites
Mucous membrane- ingestion, inhaled through respiratory system, eyes and nose, sexual contact
Adhesion and colonisation
Adhesion refers to the capability of pathogenic microbes to attach to the cells of the body using adhesion factors (adhesins)
Adhesins- specialised molecules or structures on the pathogens cell surface to bind complementary receptor sites on the host cell surface. Each organism has a specific region/cell where the pathogen will attach
Entry of pathogen
Once adhesion is successful, invasion can proceed; penetrate epithelium after attachment.
Accomplished through production of lytic substances (toxins)
- attack ground substance and basement membranes of intestinal linings
- degrading carbohydrate-protein complexes
- disrupt cell surface
Some pathogens use passive mechanisms (not related to pathogen)
- Lesions, ulcers in mucous membrane, Tissue damage from another organism’s eukaryotic internalisation pathway (endocytosis)
Toxigenicity (virulence factors)
Exotoxins - produced from the cells
- among the most lethal substances known
- associated with specific cases
- high immunogenic
- neurotoxins, cytotoxins, enterotoxins depending on mechanism of action
Endotoxins- produced from the cell surface
- LPS bound to the host
- toxic in high doses
- weakly immunogenic
- cause fever, shock, blood coagulation, weakness, diarrhoea, inflammation, intestinal haemorrhage, fibrinolysis
(- can be dead and toxins still released from outside)
Evasion of host immune system
Rely on recognition of foreign material- identify self from non-self.
Many mechanisms involved
- mimic host molecules
- create cysts
- bind to fibrinogen and complement factors in plasma
- leukocidins- cause degranulation of lysosomes
- bind to immunoglobulins by Fc end (Fc located on tail of antibody and they use this region to bind and initiate immune response. Bacteria bind here so they they are not able to initiate a response)
- produce porins to escape phagocytosis
Infection
Following invasion, successful multiplication of the pathogen leads to infection.
Infections can be described as local, focal, or systemic, depending on he extent of the infection.
Local infection: is confined to a small area of the body, typically near the portal of entry. e.g. pneumonia is confined to the lungs, ear infections, wound infections.
Focal infection: a localised pathogen, or the toxins it reduces, can spread to a secondary location. E.g. a dental hygienist nicking the gum with a sharp tool can lead to a local infection in the gum by Streptococcus bacteria of the normal oral microbiota
Systemic infection: when an infection becomes disseminated throughout the body.
Several STIs are either always systemic or can become systemic infections. e.g. HIV- affects T helper cells which causes immune system to be weakened and spreads round body.
What are the key traits required for a microbe to be pathogenic?
Transmission, adhere to host, invade host, multiply, evade host defences, cause damage to host
What do the terms pathogenic and virulent mean?
Pathogenic- ability to cause disease
Virulent- degree of pathogenicity/severity
Sexually Transmitted Infections
Are spread predominantly by unprotected sexual contact. Some STIs can also be transmitted during pregnancy, childbirth and breastfeeding and through infected blood or blood products.
More than 1 million STIs are acquired everyday worldwide, the majority of which are asymptomatic.
Each year there are an estimated 374 million new infections with 1 of 4 curable STIs: chlamydia, gonorrhoea, syphilis and trichomoniasis
Drug resistance is a major threat to reducing the burden of STIs worldwide
Types of STIs
Bacterial
- Chlamydia (most common in UK)
- Gonorrhoea
- Syphilis (2nd most common in UK)
- Non-specific urethritis
Viral
- Genital warts
- Genital herpes (least common in UK)
- HIV/AIDS
- HPV
Fungal
- Thrush (not technically an STI but usually grouped with them
Parasite
- Scabies
- Pubic lice
- Trichomonas vaginalis
Chlamydia
Bacterial infection
Gram negative (stains pink as cell wall is thinner)
Lacks peptidoglycan cell wall
Non-motile
Smallest bacterium with 2 forms
- elementary bodies (smaller) cocci
- reticulate bodies (larger) pleomorphic
- Intracellular pathogen
- Growth only in vesicles within host cells
- Do not have genes required for ATP production
- Require host cells to provide ATP
- Require the two forms for development
- Three species cause disease
- Chlamydia trachomatic - STD
- Chlamydia pneumoniae - pneumonia
- Chlamydia psittaci - from birds
Life cycle of chlamydia
All chlamydiae share a developmental cycle in which they alternate between extracellular, infectious elementary body (EB) and the intracellular, non-infectious reticulate body (RB).
0-6 hrs - EB attachment and entry by endocytosis (or break down wall using toxins)
6-12 - transition of EB to RN
12-24 - replication
24-36 - RBs reorganise back to EBs
46-48 -
48-72 - lysis or extrusion
What is the difference between EBs and RBs (chlamydia)
EB
- Small (0.2-0.1 micrometers)
- Dormant
- Resistant to environmental stress
- Infective form
- Infect through abrasions/lacerations
- Attach to limited array of cells
RB
- Larger (0.6-1.5 micrometers)
- Non-infective
- Obligate intracellular form
- Replicate by binary fission within phagosomes
Disease (chlamydia)
- Enters host cell via abrasions and lacerations then endocytose by target cell.
- EBs infect limited array of cells (that contain the receptors for EBs). Conjunctiva, cells lining the trachea, bronchi, urethra, uterus, uterine tubes, anus, rectum.
- Disease caused by inflammatory response at the infection site (inflammatory response- histamine release, vasodilation)
- Second infection causes hypersensitive immune response.
- Can result in blindness, sterility or sexual dysfunction.
- Once bacteria has entered host cell and starts to multiply a genital lesion appears.
- Lesion is relatively short lived and often missed especially in women as it develops internally.
- As infection develops lymph nodes become inflamed and headaches and flu like symptoms occur.
- In rare occasions, genital sores, constriction or the urethra and genital elephantiasis can occur.
Epidemiology of chlamydia
Bacteria can be transmitted via droplets, hands, contaminated formats or flies.
500 million people contract ocular infections every year.
Children carry pathogen in digestive and respiratory tracts (therefore signs of bacteria does not necessarily indicate abuse).
Chlamydia infects about 131 million people worldwide each year.
It is the second leading cause of more than 1 million new STIs that occur daily.
in 2019 there were 468,342 new STI diagnoses made at sexual health services in England with almost half being attracted to chlamydia.
Chlamydia symptoms
In women
- Often asymptomatic (85%)
- An unusual vaginal discharge
- Pain when urinating
- Low abdominal pain
- Bleeding between perios
- Pain during or bleeding after sex
- Immune response to second infection can lead to pelvic inflammatory disease
In men
- 75% men have symptoms
- A white/cloudy watery discharge
- Pain or burning sensation when urinating
- Testicular pain or swelling
Chlamydia diagnosis and treatment
Diagnosis- urine test or swab
Treatment- antibiotics
- Azithromycin (single dose)
- Doxycycline (7 days)
- Erythromycin (14 days)
Why are penicillin and cephalosporins not suitable for a chlamydia infection?
They are effective against gram positive bacteria and chlamydia is gram negative. They target peptidoglycan synthesis but in gram negative bacteria there is a lipopolysaccharide layer surrounding the peptidoglycan layer which prevents the penicillin attacking.
Gonorrhoea
Neisseria gonorrhoea.
Gram negative (pink stain).
Diplococci.
True pathogen.
Second most common STI.
Occurs in humans only.
Infected epithelial cells in mucosal membrane of genital, urethra, digestive tract, cervix, uterus, rectum, pharynx, mouth.
Can enter the bloodstream to cause infection of joints, heart and meninges.
Can be passed to child during childbirth and infect cornea.
Mostly transmitted sexually so can be an indication of child abuse.
