Abstract

The Staphylococcus aureus bacterium commonly known as Staph was discovered in the 1880s. Anton J. Rosenbach isolated Staphylococcus aureus which he named it “aureus” from the Latin word aurum for gold due to the yellow colonies they produced. Staph was discovered in Scotland in pus from surgical abscesses.

Staphylococcus aureus is a gram positive, non-motile, small round shaped cocci found in grape-like clusters. It belongs to the genus Staphylococcaceae. It affects all known mammalian species including humans. Staphylococcus aureus may occur commonly in the environment and is transmitted through air droplet or aerosol.

Another method of transmission is through direct contact with objects that are contaminated. Approximately 30% of healthy humans carry S. aureus at their nose, back of the throat and on their skin. S. aureus infections commonly cause painful skin and soft tissue conditions such as boils, scalded skin syndrome and impetigo.

More serious forms of S. aureus infections can progress to bacterial pneumonia and bacteria in the bloodstream both of which can be fatal. S. aureus acquired from improperly prepared food or stored food can also cause a form of food poisoning. S. aureus is one of the most resistant among all non-spore forming bacteria.

Most strains are relatively stable that is, withstand temperatures as high as 50℃ for 30 minutes. They resist a high concentration of salt (7.5-9.0% NaCl). S. aureus is associated with some enzyme production such as catalase, coagulase as well as some toxins such as hemolysins, enterotoxins, leukocidins etc. These play a key role in the pathogeneses of the organism.

CHAPTER ONE

INTRODUCTION AND LITERATURE REVIEW

1.1 Introduction

Staphylococcus aureus is a Gram positive bacterium and causative agent of wide range of infectious diseases such as skin infections, bacteremia, endocarditis, pneumonia and food poisoning. The organism was originally a leading nosocomial pathogen and afterwards, epidemiologically distinct clones emerged in community settings. S. aureus expresses number of virulence factors which help to establish infection by facilitating tissue attachment, tissue invasion and evading from host immune response.

The ability to acquire resistance to multiple antibiotic classes makes S. aureus a challenging pathogen to treat. Emergence and spread of S. aureus strains which are resistant to methicillin referred to as methicillin resistant Staphylococcus aureus (MRSA) resulted in high morbidity, high mortality and increased treatment costs.

Vancomycin remained gold standard drug to tackle these strains for years but the emergence of resistance restricted its clinical utility. Newer anti-MRSA antibiotics which were approved by the US-FDA came as respite for clinicians. However, new antibiotic discovery efforts and non-antibiotic discovery efforts and non-antibiotic approaches to take MRSA should not be diminished considering the ability of the pathogen to acquire resistance to newer drugs quickly after their introduction in clinics.

1.2 Literature review

1.2.1 Microscopic morphology

1. aureus cells are Gram positive and appear in spherical shapes. They are often in clusters resembling bunch of grapes when observed under light microscope after Gram staining. The name Staphylococcus was derived from Greek, meaning bunch of grapes (staphyle) and berry (kokkos).

1.2.2 General cultural and biochemical characteristics

7. aureus is an aerobic and facultative organism that forms fairly large yellow or white colonies on nutrient rich agar media. The yellow color of the colonies is imparted by the carotenoids produced by the organism. The term aureus is derived from Latin, which refers to the color of gold. The organism is often hemolytic in blood due to production of four types of hemolysis (α, β, γ and θ). Nearly all isolates of S. aureus produce catalase enzyme, the organism is salt tolerant which is able to grow in mannitol-salt agar medium containing 7.5% sodium chloride. The organism is catalase positive and oxidase negative.

1.2.3 Medical laboratory diagnosis

Definitive diagnosis of S.aureus infection is made by obtaining a culture from the area of suspected infection. Suspect diagnosis is based on patient’s symptoms and the health care provider’s evaluation. For the species identification, slide and tube coagulase tests, latex agglutination tests and PCR based test are used.

1.2.4 Pathogenesis

1. aureus is a widespread commensal bacterium and pathogen. Approximately 50% to 60% of individuals are intermittently or permanently colonized with S. aureus and, thus, there is relatively high potential for infections. Staphylococcus aureus causes many types of human infections and syndromes most notably skin and soft tissue infections. Abscesses are a frequent manifestation of skin and soft tissue infection and are formed, in part, to contain the nidus of infection. Polymorphonuclear leukocytes (neutrophils) are the primary host defense against S. aureus infections and a major component of S. aureus abscesses. The process of S. aureus infections involves five stages; colonization, local infection, systemic dissemination and sepsis, metastatic infections and toxinosis.

The organism is in carrier state in the anterior nares and can remain so without causing for weeks or months. The colonization proceed to infection under certain predisposing factors such as prolonged hospitalization, immune suppression, surgeries, use of invasive medical devices and chronic metabolic disease. Localized skin abscess develop when the organism is inoculated into skin from site of carriage.

The organism can enter into blood and spread systematically resulting to endocarditis, osteomyelitis, renal carbuncle, septic arthritis and epidural abscess. Without a blood stream infection, specific syndromes can occur due to extracellular toxins of S. aureus such as toxic shock syndrome (TSS), scalded skin syndrome and foodborne gastroenteritis.

1.2.5 Virulence factors

1. aureus has virulence factors which aids it attachment to host cells, breaking down the host immune shield, tissue invasion, causing sepsis and elicit toxin mediated syndromes. This is the basis for persistent staphylococcal infection without strong host immune response. The virulence of S. aureus is attributed to many factors. Some of them are implicated in lung infection and have been known for several years.

However, the information published in the recent past demonstrated a new pathogenic property related to known virulence determinants of S. aureus. Better understanding of functions and mechanisms of action of each virulence factor is important for improving prognosis of individuals suffering from pneumonia.  The virulence of S. aureus is generally considered to be multifactorial and due to the combined action of several virulence determinants. One exception is the toxinosis, such as food poisoning which are caused by toxic shock syndrome, SSSS, and staphylococcal food poisoning, which are caused by toxic shock syndrome toxin, exfoliative toxins A and B, and different staphylococcal enterotoxins respectively.

The virulence factors include; adhesins, protein-A, Panton-Valentine leukocidin (PVL), α-hemolysin toxin, chemotaxis inhibitory proteins of S. aureus (CHIPS), extracellular adherence protein (Eap), proteases, lipases, nucleases, hyaluronatelyase, phospholipase C, mtalloproteases, staphylokinase, enterotoxins, toxic shock syndrome toxin-1 (TSST-1), exfoliative toxins A and B.

1.2.6 Treatment

The history of S. aureus treatment is marked by the development of resistance to each new class of antistaphylococcal antimicrobial drugs, including the penicillins, sulfonamides, tetracyclines, glycopeptides, and others, complicating therapy. S. aureus isolates identified in 1960s were sometimes resistant to methicillin, a β-lactam antimicrobial active initially against a majority of S. aureus strains.

The MRSA isolates were resistant to nearly all β-lactam antimicrobials and were first largely confined to the health care environment and the patients who attended it. MRSA infections are treated with drugs in other antimicrobial classes. The glycopeptide drug vancomycin and in some countries teicoplanin, is the most common drug used to treat severe MRSA infections. Vancomycin was discovered from the bacterium Streptomyces orientalis and approved in 1958.

1.3 Hypothesis

Null hypothesis H0:

Staphylococcus aureus is present in foleri.

Alternative hypothesis Ha:

Staphylococcus aureus is absent in foleri.

1.4 Objectives

1.4.1 Overall objective

The overall objective was to isolate and identify S. aureus.

1.4.2 Specific objective

1. To isolate and identify S. aureus in different foleri samples.
2. Characterization of strains of S. aureus based on their hemolytic patterns.
3. To identify pathogenic strains of S. aureus.

1.5 Rationale

The basis of this experiment was to determine the prevalence of S. aureus in food sample (foleri) sold in Molyko, Buea. This was done so