Throughout history, humanity has fallen victim to pandemics of cholera, plague, influenza, typhoid, tuberculosis and other infectious maladies so widespread that few people survived into what is now considered “middle age.” As recently as the 19th century, the average life span in Europe and North America was about 50 years. It was a world in which the likelihood of dying prematurely from infectious diseases was as high as 40%, and where women routinely succumbed to infections during childbirth which are now easily curable by today's standards. In underdeveloped nations, the situation was even worse. Unfortunately, however, unlike many industrialized nations, medical conditions in many underdeveloped nations have never really improved. Indeed, in poorer nations today, infectious diseases, both major and seemingly minor, still contribute to premature death and to the ongoing misery of underprivileged populations.
The emergence of multi-resistant, or “antimicrobial agent-resistant” bacteria, has threatened the security of developed nations and further shaken the citizens of less-developed countries, and is now a worldwide concern. In many nations, antimicrobial agents are used indiscriminately, further contributing to the rise of antibiotic resistance in a variety of bacteria, including species of Enterococcus, Staphylococcus, Pseudomonas, and the Enterobacteriaceae family. The emergence of antibiotic-resistant organisms is very often a result of the over-use of broad-spectrum antimicrobial agents. There is also concern that inappropriate veterinary use of antimicrobial agents may lead to development of antibiotic resistant bacteria. In some cases, these bacteria could then, in turn, infect humans.
The diagnosis of infectious diseases has traditionally relied upon various microbiological culture methods to identify the organism responsible for an infection and then to determine the appropriate antimicrobial treatment for the patient. These methods continue to be important for analysis, despite recent advances in molecular and immunological diagnostics. While the development of rapid and automated methods has served to increase the efficiency of microbiological analysis, traditional quantitative culture methods remain critical for definitive diagnosis of infections. See, Baron & Finegold, Diagnostic Microbiology, 8th ed. C. V. Mosby, (1990), p. 253. Further, these traditional methods are even more valuable in countries unable to afford newer methods, such as automated identification and susceptibility-testing methods. In addition, many areas of the world are devoid of adequate clinical microbiology facilities capable of providing access to newer diagnostic methods. Indeed, in some cases, even traditional culture-based methods are only narrowly available.
Traditional culture-based diagnostic methods share a general set of method steps. A first group of these steps involves the collection and transport of a specimen. The specimen must be material from the actual infection site. Once collected, it is necessary to maintain the sample as near to its original state as possible with minimum deterioration. Transport systems often consist of a protective container, transport medium and a culture swab. A problem with the use of a holding or transport medium is that it may jeopardize the recovery of certain strains. A major task is to reduce the time delay between collection of specimens and inoculation onto microbiological culture media. The transport container is constructed to minimize hazards to specimen handlers. It is best to minimize adverse environmental conditions, such as rapid changes in pressure, exposure to extremes of heat and cold or excessive drying. The transport of fluid specimens to the laboratory must be done as quickly as possible. It is recommended that a 2-hour maximum time limit be imposed between collection and delivery of specimens to the laboratory. This limit poses a problem for specimens collected any distance from a clinical microbiology laboratory.
In addition to the above difficulties, under some conditions, traditional microbiological culture media suffer from several weaknesses. First, satisfactory microbiological culture media must generally contain many components to successfully support bacterial life. More specifically, satisfactory media must include available sources of water, vitamins, inorganic phosphate and sulfur, trace metals, carbon and nitrogen. These needs may be supplied from a number of sources. In addition, various media may include agents which selectively allow the growth of specific organisms while preventing the growth of others. Media may often include compounds that enhance the ability of a user to identify the bacteria growing thereon. The following is a list of common media constituents with their sources in parenthesis: (1) Amino-nitrogen (peptone, protein hydrolysate, infusions and extracts), (2) Growth factors (blood, serum, yeast extract or vitamins, NAD), (3) Energy sources (sugar, alcohols, and carbohydrates), (4) Buffer salts (Phosphates, acetates and citrates), (5) Mineral salts and metals (phosphate, sulfate, magnesium, calcium, iron), (6) Selective agents (chemicals, antimicrobials and dyes), (7) Indicator dyes (phenol red, neutral red), and (8) Solidifying agents (agar, gelatin, alginate, silica gel, etc.).
A selection of the appropriate solid culture media for microbiological test(s) is generally made according to the particular specimen type. Several hundred standard culture media are commercially available. Various culture media have been developed to serve specific purposes, including the identification of bacteria and antibiotic susceptibility testing. One medium used in antibiotic susceptibility testing is Mueller Hinton agar. The media used as identification testing media can generally be divided into five groups: enriched media, differential media, selective media, differential-selective media, and single purpose media. Enriched media have special additives to support pathogens having fastidious growth needs. Examples of enriched media include sheep blood agar and brain heart infusion broth. Differential media allows the differentiation of groups of microorganisms based on color changes of an indicator (sensitive to a property such as pH) in the culture medium that take place as a result of biochemical reactions associated with microorganism growth. Separating organisms that ferment the sugar lactose, for example, from those that do not, is one example of the utility of differential media.
Selective media support the growth of certain microorganisms of interest while suppressing the growth of others. Azide blood agar, Columbia CNA agar with blood and Phenylethanol agar are examples. Gram-positive organisms grow on these media whereas gram-negative organisms do not. Differential-selective media combine the characteristics of both selective media and differential media, thus allowing the selective growth and rapid differentiation of major groups of bacteria. These media are widely used in tests for gram-negative bacilli (rods). MacConkey and Hektoen media are examples. Single-purpose media isolate one specific type of microorganism. Bile esculin azide agar is an example of this media. Enterococcus and group D streptococcus grow and cause the formation of a dark brown or black complex in the agar.
In modern microbiology laboratories, every attempt is made to use well-trained personnel, working under close supervision, in the processing of specimens. Errors or misjudgments made during laboratory processing, such as improper choice of culture media, can negate all the expertise one may apply in later processing steps such as the reading and interpretation of cultures. Expert microbiologists may often be caught short in making definitive diagnoses because of the selection and use of inadequate or incorrect media in culturing a specimen.
The equipment required for the primary inoculation of specimens includes several microbiological agar-based media plates and a nichrome or platinum inoculating wire or loop. Plastic disposable loops are also available. Plates currently used in the field generally have a shelf life of from one to two months. Specimens are “streaked out” on the surface of the plates to spread the microorganisms across the surface of the solid culture medium. This results in isolated colonies.
As the isolated microorganisms grow on the solid medium, they form a mass called a colony. This mass of cells originated from a single cell and now may consist of hundreds of thousands of cells. These colonies have distinct characteristics that are a clue in the process of identifying the microorganism (see FIG. 9). The microscopic examination of a suspension of bacteria from a colony reveals (a) cellular morphology, (b) cellular arrangement, and (c) motility. These features (See FIG. 12) add additional pieces to the ID puzzle. A gram stain of the sample may also assist the analyst in getting closer to a characterization of the organism. The gram stain is not foolproof however, and can be occasionally misleading because the staining is frequently dependent upon the age of the colony.
Current microbial testing methods call for initial isolation and identification of the organism first and then, if deemed appropriate, i.e. where a pathogen is identified, performing an antimicrobial susceptibility test. In addition, the analyst must decide which microorganism is responsible for the clinical disease in mixed cultures. There are a number of different ways of doing antimicrobial susceptibility testing (AST). Two of them are disk-diffusion and micro dilution.
In recent years, there has been a trend toward the use of commercial broth micro dilution and automated instrument methods instead of the disk-diffusion procedure. However, there may be renewed interest in the disk-diffusion test because of its inherent flexibility in drug selection and low cost. The availability of numerous antimicrobial agents and the diversity in antibiotic formularies in different institutions has made it difficult for manufactures of commercial test systems to provide standard test panels that fit every facility's needs. Thus, the inherent flexibility of drug selection provided by the disk-diffusion test is an undeniable asset of the method. It is also one of the most established and best proven of all AST tests and continues to be updated and refined through frequent National Committee for Clinical Laboratory Standards (NCCLS) publications. Furthermore, clinicians readily understand the qualitative interpretive category results of susceptible, intermediate, and resistant provided by the disk test. It is an ideal method when doing manual diagnostic microbiology
A distinct disadvantage of the above prior art is the total time that it takes from obtaining the culture through performing ID and AST. At least three days transpire before results are available. Another disadvantage is the expense to process the specimen using prior art. A further disadvantage of the prior art is the number of steps involved in performing the tests, which increases the likelihood of human error.
A further disadvantage is raised by the limited shelf life of the agar-based microbiology media that is currently used in the art. Specifically, most currently-available agar-based media have a shelf life of from about one to about two months at most. One problem which reduces the shelf life of such media is syneresis, a condition in which the liquid component of the agar media separates from the gel component. This dramatically reduces the utility of the media by segregating the moisture and nutrients needed in all portions of the agar in a liquid phase, rendering the agar uneven in its ability to support sample growth. This restricts the ability of facilities to maintain an inventory of suitable media and complicates the manufacture, distribution, and sale of diagnostic kits utilizing agar-based media currently known and used in the art.
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