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Prokaryote Structureand the Gram Stain
By ScholarRx
Updated August 30, 2022
access_time21 min
Learning Objectives (3)
After completing this brick, you will be able to:
● Describe the structure of prokaryotic cells.
● 1
● Discuss the physiologic niche of bacteria and their growth
characteristics.
● 2
● Describe the staining characteristics and classification and
identification of bacteria.
● 3
cableCASE CONNECTION
On your first Friday night in a small hospital emergency department,
you treat a student from the local college. KD has had a febrile illness
for almost 2 days. “First I just felt tired and generally sick. Then the
headache started, and now my neck hurts when I bend it.” You are
already thinking about admission and treatment for possible
meningitis, and you start all the usual measures—lab tests, IV fluids,
lumbar puncture, and empiric antibiotics. You ask the head nurse how
long it will take to get the Gram stain results on KD’s CSF. “Not until
Monday, I think. It’s the weekend,” she replies. You type “how to do
gram stain” into your favorite search engine.
,How will you explain your need to have the Gram stain results sooner
than later? Consider your answer as you read, and we’ll revisit KD at
the end of the brick.
GO TO CONCLUSION arrow_downward
What Are Prokaryote Structure and
the Gram Stain?
Bacteria are members of a unique taxonomic kingdom consisting of
prokaryotic unicellular organisms. Prokaryote is a term from ancient
Greek meaning “before the kernel.” The kernel in this case is a
nucleus, which prokaryotes lack. Prokaryotes also do not have any
membrane-bound organelles. In fact, many of the organelles found in
eukaryotes—like an endoplasmic reticulum, mitochondria, Golgi
apparatus, lysosomes, and peroxisomes—are completely absent in
prokaryotes.
Bacteria first began to be identified by a “defective method.” Or so its
Danish inventor, a recent medical school grad named Hans Christian
Gram, deemed it in 1884. Gram was working with lung tissue from
cadavers who had died of infections from Streptococcus pneumoniae
and Klebsiella pneumoniae when he discovered that those organisms
reacted differently to certain substances under the microscope, and—
voilà—the Gram stain was born, to identify gram-positive bacteria.
The defect he mentioned was overcome by German pathologist Carl
Weigert, who added a final step to Gram’s procedure and gave us the
method to identify gram-negative bacteria. We’re still using the same
techniques more than 130 years later!
,What Is the Structure of Prokaryotes?
Without a nucleus, the bacterial deoxyribonucleic acid
(DNA)/chromosome is stored in the cytoplasm. Unlike eukaryotic
linear chromosomes, bacterial chromosomes are circular, without free
ends of DNA. In addition, bacterial chromosomes do not contain
nonexpressed regions (introns) like eukaryotes do. Reproduction is
typically asexual and results in daughter cells that are identical to the
original.
Although bacterial anatomy can vary quite a bit (despite their tiny
size), we’ll look at a few common features that they all share.
Appendages
These structures are useful for locomotion and avoidance of
micropredation (eg, immune system cells).
Flagella. These elongated protein filament cylinders are anchored
inside the cell wall and plasma membrane of predominantly gram-
negative bacteria. Whip-like motion of the flagellum propels the
bacterium, and the arrangement of flagella can be quite useful in
bacterial identification (eg, can be at one or both ends, thus polar like
Pseudomonas aeruginosa, or all around, so peritrichous, as in
Salmonella spp). In addition, the flagellum motor serves as a response
regulator in bacterial chemotaxis. Chemotaxis is the movement
toward or away from certain chemicals in the environment. For
example, chemotaxis could cause a bacterium to move closer to
nutrients or “flee” from toxic compounds. The efficiency of flagella-
mediated chemotaxis varies among bacterial species.
Pili. Pili or fimbriae appear as tiny hairs around a bacterium.
Although also present in gram-positive bacteria, these appendages are
more common, or at least better understood, among gram-negative
, species. (Gram-positive and gram-negative bacteria will be
differentiated a little more later on.) Adhesins on the tips of pili play
an integral role in adhesion, allowing the bacterium to attach to
surfaces in the environment or to other organisms such as to the
human intestinal lining. Larger fimbriae (aka, R-pili) are involved in
drug resistance transfer by bacterial conjugation.
Cytoplasm
As mentioned, prokaryotes lack membrane-bound organelles,
including nuclei. For example, although bacteria have free ribosomes,
they do not possess an endoplasmic reticulum, a membranous
organelle found in eukaryotes that is rich in ribosomes. Most of the
metabolic processes that would be carried out by cytoplasmic
organelles in eukaryotes are therefore relegated to the inner
membrane in prokaryotes.
Cell Envelope
The prokaryotic cell envelope contains the barriers between the inner
cell and the environment (Figure 1). It functions to maintain cell
shape and provide structural integrity to prevent lysis from osmosis,
and it plays a minor role in locomotion by its selectively permeable
structure. The constituents of the envelope also determine whether
bacteria stain gram positive or gram negative. Depending on which,
the cell envelope may contain a capsule, an outer membrane with
proteins, periplasm, a cell wall, or a cytoplasmic membrane.
Surface Constituents
Gram-positive bacteria have a thick outer cell wall and cytoplasmic
lipid membrane. Gram-negative bacteria have an outer lipid
membrane, a cell wall, and an inner or cytoplasmic lipid membrane.
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