De fleste af os er bekendt med de mikroorganismer, der forårsager sygdomme (patogene mikroorganismer), som, skønt de er af stor betydning, kun udgør få hundrede af de 10.000-vis af forskellige arter.

De andre mikroorganismer spiller også en rolle i biosfæren. På grund af deres store antal, deres forskellige metaboliske aktiviteter og deres evne til at vokse i forskellige habitater, er de ansvarlige for omdannelse og genbrug af mange organiske og uorganiske stoffer på globalt plan.

Mikroorganismer. Mikroorganismerne er placeret i 3 forskellige kategorier.

Eukaryote mikroorganismer - organismer med eukaryot celle struktur, hvilket betyder, at de har ben cellestruktur magen til plante- og dyrcelle. ette er relativt store celler, gennmsnitlig 10-20 myi diameter med en kompleks indre organisation. Deling af nucleus sker ved mitose og meios.

Eukaryote mikoorganismer placeres i 3 forskellige grupper:

- Eubacteria som inkluderer de fleste almindelige bakterier

- Archaebacteria - en lille gruppe af bakterier med helt specielle karakteristika

Acellulære mikroorganismer, Viruses (Vira) mangler celle struktur og består af en proteinkappe eller skal som omgiver arvematerialet. Vira er obligat intracellulære parasitter hos planter, dyr og andre cellulære organismer. De kan kun reproducere i levende værtsceller

 lack cell structure and are composed of

a protein coat or shell that encloses a nucleic acid chromosome - in some cases

a lipid envelope surrounds the entire structure. The intact form of the virus is

Viruses are obligate intracellular parasites of plants, animals and cellular

microorganisms. They can only reproduce in living host cells. They do so by

an assembly process that does not involve division of the virion. Instead

virus proteins and nucleic acids are synthesized separately. The components

are assembled to a coat into which the chromosomes is inserted and, in the

final stages, the assembled intact virus (virion) are released from the host.

Microorganisms and Evolution: In recent years a clearer picture of the

evolution of cellular microorganisms, but not viruses, has been put forth. In

this scheme, all organisms are thought to have evolved from ancestral,

unicellular organisms (progenote) some 3.8 billion years ago.

- The progenote is thought to have evolved to form 3 distinct groups

(archaebacteria) and the Eucarya (eukaryotic organisms) - see Figure 19.3,

p. 396.

- The eubacteria and archaebacteria are prokaryotic while the 3rd domain

consists entirely of eukaryotes.

- The eukaryotic domain further evolved into the eukaryotic microorganisms

(protozoa, fungi, and algae) and the plants and animals. The term

microorganism, therefore, does not refer to a specific evolutionary group of

organisms but to organisms in all 3 of the principle domains of organisms.

This is surprising since, in terms of evolution, the eukaryotic

microorganisms are much more closely related to plants and animals than

they are to prokaryotes.

Definition of cellular microorganism: How, then, are microorganisms

defined? It does not refer to an group related to each other through evolution.

Although most microorganisms are unicellular and, therefore, invisible to the

naked eye, size does not define them since some filamentous and multicellular

microorganisms, e.g., algae (seaweeds such as kelp) and fungi (mushrooms),

plants and animals (transparencies).

- The first picture shows the cells that make up the structure of a leaf. The

cells are differentiated into many different types, each type carrying out a

specific function, e.g., epidermis, photosynthesis, tubules, etc. The cells are

highly organized to make up the organ (leaf).

- The second picture shows a brown alga which has a multicellular, plant-like

structure. In contrast to the leaf, the cells are more or less the same, i.e.,

limited differentiation of cells into different types or into different organs.

- The third picture shows a green alga. It is clear that the organism lacks the

typical cell structure and consists of a cytoplasm in which many nuclei are

distributed, i.e., multinucleate or coenocytic structure.

Microorganisms can therefore be defined as organisms that, unlike plants and

- It is important to note that some microorganisms can undertake

differentiation, but that this ability is limited and not as pronounced as in

plants and animals.

Antonie von Leeuwenhoek (1632-1723): Although multicellular and

coenocytic forms of algae and molds attain macroscopic sizes and are readily

visible, unicellular microorganisms are microscopic and cannot be seen with

the naked eye - it can only see objects with dimensions greater than 0.1 mm

They were not seen until the development of the single lens (simple)

microscope by Leeuwenhoek which could magnify objects 50-300x. He was the

first to see cells (sperm, erythrocytes, etc.) and to describe capillary circulation

the unicellular microorganisms.

1 . Animalcules were present in almost every sample he studied, i.e., they were

ubiquitous.

2 . Different types of animalcules were identified. Forms are recognizable as

belonging to different species.

Resurgence in Study of Microorganisms: No one else was able to make

these lenses, so that the study of microorganisms was abandoned after

Leeuwenhoek's death. It was not till the 1800's that interest in microorganisms

was rekindled. This was due to 2 events:

1 . Perfection of the compound microscope with multiple lens systems

simplified the observation of microorganisms. These microscopes are

essentially the same type that are used in the lab.

that life forms could arise spontaneously from inert, non-living material.

Spontaneous Generation: Organic matter (meat, plant material) was placed

in water to make a broth, referred to as organic infusions, which was rich in

nutrients that could support the growth of microorganisms. Upon standing for

a day or two the clear infusion became cloudy or turbid due to the dense

growth of microorganisms, likely bacteria. To produce visible turbidity, they

infusion. The appearance of microorganisms could be due to:

1 . the growth of microorganisms that were spontaneously formed within the

infusion.

2 . growth of organisms that were already present or accidentally introduced in

the infusion was being handled, i.e., due to the growth of contaminating

microorganisms (contaminants).