--- title: Bacterial Infections and Eological Agents videoId: zulwFmL2jlE ---
From: [[marrowmed]] <br/>
This article discusses various bacterial infections, their causative agents, diagnostic methods, and related clinical phenomena, as presented in a microbiology MCQ discussion for the INI CET November 2024 exam <a class="yt-timestamp" data-t="00:04:00">[00:04:00]</a>.
Key Topics in Bacteriology
The exam highlighted several important areas within microbiology, including: * Sterilization and disinfection <a class="yt-timestamp" data-t="00:39:00">[00:39:00]</a> * Arrangement and morphology of bacteria <a class="yt-timestamp" data-t="00:47:00">[00:47:00]</a> * [[General Bacteriology and Culture Techniques | General Bacteriology]] <a class="yt-timestamp" data-t="00:52:00">[00:52:00]</a> * Culture media <a class="yt-timestamp" data-t="00:56:00">[00:56:00]</a> * Antibiotic sensitivity <a class="yt-timestamp" data-t="00:59:00">[00:59:00]</a> * Etiological agents of various diseases <a class="yt-timestamp" data-t="01:15:00">[01:15:00]</a> * [[Sexually Transmitted Infections and Urethritis | Urethritis]] <a class="yt-timestamp" data-t="01:37:00">[01:37:00]</a> * Trauma-acquired infections <a class="yt-timestamp" data-t="01:31:00">[01:31:00]</a>
Sterilization and Disinfection Techniques
Bacteriological Loops
* Bacteriological loops, used for inoculating culture media, are typically sterilized by placing them in a flame until they become red hot <a class="yt-timestamp" data-t="02:27:00">[02:27:00]</a>. * While autoclaving is a sterilization method, it is impractical for frequent, repeated sterilization of inoculation loops during culture work <a class="yt-timestamp" data-t="02:53:00">[02:53:00]</a>. * Loops are often made of expensive materials like platinum or nichrome <a class="yt-timestamp" data-t="03:07:00">[03:07:00]</a>.
Other Disinfection Methods
* Passing slides and cover slips through a flame three times is a method for disinfection <a class="yt-timestamp" data-t="03:31:00">[03:31:00]</a>.
Bacterial Morphology and Staining
Different bacteria exhibit distinct morphologies and arrangements when viewed under a microscope, which aids in their identification.
Gram-Positive Cocci
* **_Streptococcus pyogenes_**: Appears as Gram-positive cocci in chains <a class="yt-timestamp" data-t="03:56:00">[03:56:00]</a>. It is the most common cause of necrotizing fasciitis <a class="yt-timestamp" data-t="04:09:00">[04:09:00]</a>. * **_Staphylococcus_**: Arranged as Gram-positive cocci in clusters <a class="yt-timestamp" data-t="04:17:00">[04:17:00]</a>. * **_Micrococcus_**: Appears as Gram-positive cocci in tetrads (sets of four) <a class="yt-timestamp" data-t="04:20:00">[04:20:00]</a>. * **_Enterococcus_**: Arranged in pairs, often at an angle to each other <a class="yt-timestamp" data-t="04:29:00">[04:29:00]</a>.
Gram-Positive Rods
* **_Nocardia_**: * Causes infections following blunt trauma, especially in farmers <a class="yt-timestamp" data-t="04:36:00">[04:36:00]</a>. * Microscopy of wound exudate shows weakly acid-fast beaded filamentous rods <a class="yt-timestamp" data-t="04:44:00">[04:44:00]</a>. * These appear as red-colored rods against a blue background in an acid-fast stain <a class="yt-timestamp" data-t="05:01:00">[05:01:00]</a>. * It belongs to the *Actinomycetales* family, which consists of Gram-positive, slender, branching rods <a class="yt-timestamp" data-t="05:37:00">[05:37:00]</a>. * _Nocardia_ is weakly or partially acid-fast, meaning it stains with a lower concentration of sulfuric acid (0.5-1%) as a decolorizer <a class="yt-timestamp" data-t="06:13:00">[06:13:00]</a>. * It is aerobic <a class="yt-timestamp" data-t="06:33:00">[06:33:00]</a> and causes exogenously acquired infections, as it is a normal environmental saprophyte <a class="yt-timestamp" data-t="06:43:00">[06:43:00]</a>. * **_Actinomyces_**: * Morphologically similar to _Nocardia_ on Gram stain <a class="yt-timestamp" data-t="06:08:00">[06:08:00]</a>. * Unlike _Nocardia_, it is not acid-fast <a class="yt-timestamp" data-t="06:27:00">[06:27:00]</a>. * It is anaerobic <a class="yt-timestamp" data-t="06:37:00">[06:37:00]</a> and causes endogenous infections, as it is part of the normal flora in the oral cavity, GI tract, or female vagina <a class="yt-timestamp" data-t="06:49:00">[06:49:00]</a>. * **_Bacillus anthracis_**: A Gram-positive rod, but does not show slender morphology or branching, and is not acid-fast <a class="yt-timestamp" data-t="07:13:00">[07:13:00]</a>. * **_Listeria_**: A Gram-positive rod, but not acid-fast, not branching, and not filamentous <a class="yt-timestamp" data-t="07:26:00">[07:26:00]</a>. * **_Mycobacterium tuberculosis_ (MTB)**: * Is acid-fast, typically with 20% sulfuric acid as a decolorizer, but does not have the filamentous morphology of _Nocardia_ <a class="yt-timestamp" data-t="07:34:00">[07:34:00]</a>. * The easiest culture medium for its isolation is Lowenstein-Jensen's (LJ) medium, an egg-containing green medium <a class="yt-timestamp" data-t="07:55:00">[07:55:00]</a>. * Malachite green in LJ medium acts as a selective agent, inhibiting unwanted bacterial growth <a class="yt-timestamp" data-t="08:11:00">[08:11:00]</a>. * Due to a long generation time (14-15 hours), MTB colonies form in 3-8 weeks <a class="yt-timestamp" data-t="08:31:00">[08:31:00]</a>. * Colony morphology is described as "rough, tough, and buff" (off-white) <a class="yt-timestamp" data-t="08:40:00">[08:40:00]</a>.
[[Microbial phenomena and tests | Antibiotic Sensitivity Testing]]
E-Test (Epsilometer Test)
* A test used for antibiotic sensitivity testing <a class="yt-timestamp" data-t="09:10:00">[09:10:00]</a>. * Involves placing plastic strips impregnated with a graded concentration of antibiotic onto a Mueller-Hinton agar plate inoculated with a bacterial lawn <a class="yt-timestamp" data-t="09:20:00">[09:20:00]</a>. * After overnight incubation, ellipsoidal zones of inhibition are observed <a class="yt-timestamp" data-t="09:40:00">[09:40:00]</a>. * The point where the zone of inhibition intersects the strip indicates the Minimum Inhibitory Concentration (MIC) <a class="yt-timestamp" data-t="09:46:00">[09:46:00]</a>. * Advantages: Determines the MIC (quantitative test) <a class="yt-timestamp" data-t="09:57:00">[09:57:00]</a>. * Disadvantage: Expensive <a class="yt-timestamp" data-t="10:03:00">[10:03:00]</a>.
Kirby-Bauer Disc Diffusion Test
* More commonly used due to its lower cost and ease <a class="yt-timestamp" data-t="10:14:00">[10:14:00]</a>. * Uses discs impregnated with a single, standardized concentration of antibiotic <a class="yt-timestamp" data-t="10:27:00">[10:27:00]</a>. * Zone of inhibition diameter is measured to determine if the bacterium is sensitive, intermediate sensitive, or resistant <a class="yt-timestamp" data-t="10:37:00">[10:37:00]</a>. * Disadvantage: Does not determine the exact MIC (qualitative test) <a class="yt-timestamp" data-t="10:51:00">[10:51:00]</a>.
Specific Bacterial Infections and Etiological Agents
Necrotizing Fasciitis
* Most commonly caused by _Streptococcus pyogenes_ (Group A beta-hemolytic Streptococcus) <a class="yt-timestamp" data-t="04:09:00">[04:09:00]</a>.
Donovanosis (Granuloma Inguinale)
* Caused by _Klebsiella granulomatis_ (formerly _Calymmatobacterium granulomatis_) <a class="yt-timestamp" data-t="14:52:00">[14:52:00]</a>. It is a [[Sexually Transmitted Infections and Urethritis | genital ulcer disease]] <a class="yt-timestamp" data-t="14:29:00">[14:29:00]</a>.
Oroya Fever (Carrión's Disease)
* Reported mainly from South America <a class="yt-timestamp" data-t="15:00:00">[15:00:00]</a>. * Transmitted by the bite of a sandfly <a class="yt-timestamp" data-t="15:07:00">[15:07:00]</a>. * Caused by _Bartonella bacilliformis_ <a class="yt-timestamp" data-t="15:10:00">[15:10:00]</a>.
Rat Bite Fever
* In the Western world, caused by _Streptobacillus moniliformis_ <a class="yt-timestamp" data-t="15:36:00">[15:36:00]</a>. * In Asia, caused by _Spirillum minus_ <a class="yt-timestamp" data-t="15:39:00">[15:39:00]</a>.
Pinta
* Caused by _Treponema carateum_ <a class="yt-timestamp" data-t="15:21:00">[15:21:00]</a>.
Waterborne Diseases
* Hepatitis A is primarily a waterborne disease <a class="yt-timestamp" data-t="15:46:00">[15:46:00]</a>. Hepatitis A and E viruses are transmitted via the fecal-oral route (waterborne) <a class="yt-timestamp" data-t="15:56:00">[15:56:00]</a>.
Diarrhea in Children
* The most common cause of diarrhea in children under 5 years of age is [[Parasitic and viral diseases | Rotavirus]] <a class="yt-timestamp" data-t="23:15:00">[23:15:00]</a>. * _Enteropathogenic E. coli_ (EPEC) can cause infantile diarrheas and is the most common bacterial cause <a class="yt-timestamp" data-t="23:35:00">[23:35:00]</a>. * Diarrhea complicated by Hemolytic Uremic Syndrome (HUS), characterized by hematuria and thrombocytopenia, is most commonly caused by _E. coli O157:H7_ (an enterohemorrhagic _E. coli_ (EHEC)) <a class="yt-timestamp" data-t="24:17:00">[24:17:00]</a>.
[[Sexually Transmitted Infections and Urethritis | Urethritis]] Etiology
* Common causes of urethritis include _Chlamydia trachomatis_ (serovars D-K, most common cause) <a class="yt-timestamp" data-t="25:24:00">[25:24:00]</a>, _Neisseria gonorrhoeae_ <a class="yt-timestamp" data-t="25:30:00">[25:30:00]</a>, _Mycoplasma genitalium_ <a class="yt-timestamp" data-t="25:33:00">[25:33:00]</a>, and _Trichomonas vaginalis_ <a class="yt-timestamp" data-t="25:36:00">[25:36:00]</a>. * _Gardnerella vaginalis_ causes bacterial vaginosis, not urethritis <a class="yt-timestamp" data-t="25:39:00">[25:39:00]</a>.
Other Relevant Tests in Bacteriology
CAMP Test (Christie-Atkins-Munch-Petersen Test)
* Used for the identification of Group B beta-hemolytic _Streptococcus_ (_Streptococcus agalactiae_) <a class="yt-timestamp" data-t="11:33:00">[11:33:00]</a>. * It shows enhanced zones of beta hemolysis (arrowhead hemolysis) when _S. agalactiae_ is streaked perpendicular to _Staphylococcus aureus_ <a class="yt-timestamp" data-t="11:47:00">[11:47:00]</a>. * Also positive for _Listeria monocytogenes_ <a class="yt-timestamp" data-t="12:10:00">[12:10:00]</a>.
Elek Gel Precipitation Test
* Used for determining the toxigenicity of _Corynebacterium diphtheriae_ <a class="yt-timestamp" data-t="12:20:00">[12:20:00]</a>. * Involves a filter paper strip impregnated with diphtheria antitoxin placed on a medium, with test isolates streaked at right angles <a class="yt-timestamp" data-t="12:28:00">[12:28:00]</a>. * Toxigenic strains of _C. diphtheriae_ produce diphtheria toxin, forming lines of precipitate where toxin and antitoxin bind <a class="yt-timestamp" data-t="12:56:00">[12:56:00]</a>. * This is a double diffusion in two dimensions test, also known as the Ouchterlony test <a class="yt-timestamp" data-t="13:17:00">[13:17:00]</a>.
Latex Agglutination Test
* Latex particles coated with specific antibodies are mixed with a patient's specimen to detect antigens <a class="yt-timestamp" data-t="13:49:00">[13:49:00]</a>. * Example: Detecting capsular antigen of [[Fungal and bacterial infections | Cryptococcus]] in CSF or blood by coating latex particles with anti-cryptococcal antibodies <a class="yt-timestamp" data-t="14:09:00">[14:09:00]</a>.