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Sexually Transmitted Infections (STIs): Microbiology and Clinical Features

Introduction to Sexually Transmitted Infections

Sexually transmitted infections (STIs) are infectious diseases primarily spread through sexual contact. They are caused by a variety of microorganisms, including bacteria, viruses, fungi, and protozoa, and can affect multiple anatomical sites.

• Definition: STIs are infections transmitted via sexual activity, affecting mucocutaneous and genital tissues.

• Agents: Bacteria, viruses, fungi, protozoa, and occasionally unidentified microorganisms.

• Portals of Entry: Mouth, genital area, urinary opening (meatus), rectum, skin.

Organisms Causing Sexually Transmitted Diseases (STDs)

Major Microbial Agents and Associated Diseases

Several microorganisms are responsible for common STDs, each with distinct clinical presentations and complications.

• Condylomata acuminata: Caused by Human papillomavirus (HPV).

• Genital herpes: Caused by Herpes simplex virus (HSV).

• Molluscum contagiosum: Caused by a poxvirus.

• Chancroid: Caused by Haemophilus ducreyi.

• Granuloma inguinale: Caused by Klebsiella granulomatis.

• Lymphogranuloma venereum (LGV): Caused by Chlamydia trachomatis (L1-L3 types).

Types of Herpesviruses and High-Risk HPV Strains

Herpesviruses Causing STDs

Herpesviruses are classified into neurotropic and lymphotropic groups, with several types implicated in genital infections.

• Neurotropic α-group:

  • Herpes simplex virus type 1 (HSV-1): Usually oral, can cause genital lesions.

  • Herpes simplex virus type 2 (HSV-2): Primarily genital herpes.

  • Varicella-zoster virus (VZV): Causes chickenpox and shingles.

• Lymphotropic β/γ-group:

  • Cytomegalovirus (CMV)

  • Epstein-Barr virus (EBV)

  • Human herpesvirus type 8: Linked to Kaposi sarcoma.

High-Risk HPV Strains

• Some HPV types (e.g., 16, 18) are associated with cervical, vulvar, vaginal, anal, and oropharyngeal cancers.

• Transmission occurs via sexual contact; persistent infection increases risk for complications.

• Prevention: HPV is preventable by vaccine.

• Complication: HPV can lead to cervical cancer.

Pathogenesis and Diagnosis of Genital Herpes

Stages of Genital Herpes Infection

Genital herpes is characterized by recurrent lesions due to the ability of the virus to establish latency in nerve cells.

• Replication: Virus multiplies in skin or mucous membranes at the site of infection.

• Nerve Involvement: Virus travels along nerve pathways to the sacral dorsal root ganglia, where it becomes latent.

• Latency: Virus resides in nerve cells, remaining dormant until reactivation.

• Recurrence: Reactivation leads to new outbreaks; immune response affects severity and frequency.

Diagnosis

• Based on symptoms (pain, itching, blisters).

• Viral culture or PCR testing from lesion samples confirms diagnosis.

Sexually Transmitted Vaginal Infections

Major Vaginal Infections and Their Causes

Vaginal infections can be caused by fungi, protozoa, or bacteria, each with unique pathogenesis and clinical features.

Infection	Agent	Key Features
Candidiasis (Yeast Infection)	Candida albicans (fungus)	Itching, discharge; often due to antibiotic use, hormonal changes, or pregnancy
Trichomoniasis	Trichomonas vaginalis (protozoan)	Frothy discharge, irritation; sexually transmitted
Bacterial Vaginosis (BV)	Mixed bacterial flora	Imbalance of normal vaginal bacteria; thin discharge

Causes of Candidiasis

• Antibiotic therapy suppresses protective bacterial flora.

• High hormone levels (pregnancy, oral contraceptives) increase vaginal glycogen stores.

Trichomoniasis

• Anaerobic protozoan infects vaginal mucosa, causing irritation and discharge.

Bacterial Vaginosis

• Imbalance in normal vaginal bacteria leads to infection.

STDs Infecting Male and Female Genital and Extragenital Structures

Chlamydia Infections

Chlamydia trachomatis is the most common bacterial STD in the U.S., often presenting with mild or no symptoms.

• Symptoms: Mild or no symptoms; burning during urination, discharge, pelvic pain.

• Complications: Pelvic inflammatory disease (PID), infertility, ectopic pregnancy, newborn eye infections.

• Treatment: Antibiotics (azithromycin or doxycycline).

Gonorrhea

Neisseria gonorrhoeae infects the genitourinary tract, eyes, oropharynx, anorectum, or skin.

• Symptoms: Burning urination, yellow or white discharge, pelvic pain, sore throat if oral.

• Complications: PID, infertility, newborn conjunctivitis, joint infection (disseminated gonococcal infection).

• Treatment: Dual antibiotic therapy (ceftriaxone + azithromycin).

Syphilis

Treponema pallidum causes syphilis, a disease with distinct clinical stages and severe complications if untreated.

• Primary Stage: Painless sore (chancre).

• Secondary Stage: Rash, fever, swollen lymph nodes.

• Tertiary Stage: Organ and nervous system damage, including gummas (tumor-like growths), heart problems, and brain/nerve damage.

• Complications: Blindness, heart damage, congenital syphilis in infants.

• Treatment: Penicillin injection.

Summary Table: Major STDs, Agents, Symptoms, and Treatments

Disease	Agent	Symptoms	Complications	Treatment
Chlamydia	Chlamydia trachomatis	Mild/no symptoms, burning urination, discharge	PID, infertility, ectopic pregnancy, newborn eye infection	Azithromycin or doxycycline
Gonorrhea	Neisseria gonorrhoeae	Burning urination, discharge, pelvic pain	PID, infertility, conjunctivitis, joint infection	Ceftriaxone + azithromycin
Syphilis	Treponema pallidum	Chancre, rash, fever, lymphadenopathy	Organ damage, blindness, congenital syphilis	Penicillin injection
Genital Herpes	Herpes simplex virus	Painful blisters, itching	Recurrent outbreaks	Antiviral therapy (e.g., acyclovir)
HPV	Human papillomavirus	Warts, often asymptomatic	Cervical cancer	Vaccine, removal of warts

Key Definitions and Concepts

• Pelvic Inflammatory Disease (PID): Infection of female reproductive organs, often caused by chlamydia or gonorrhea.

• Congenital Infection: Infection passed from mother to child during pregnancy or delivery.

• Latency: Dormant state of a virus within host cells, with potential for reactivation.

Review Questions

• What are agents that cause STDs?

• What are portals of entry for STDs?

• What does HPV cause?

• What is candidiasis?

• What is trichomoniasis?

• What are symptoms and complications of chlamydia?

• What are symptoms and complications of gonorrhea?

• What are primary and secondary stages of syphilis?

• What are complications of syphilis?

Additional info: These notes expand upon the original slides by providing definitions, clinical context, and structured tables for exam preparation.